RES EAR C H PUB Lie A T ION N 0.20 Impact of Elevated Nutrients in the Great Barrier Reef Claudia Baldwin Great Barrier Reef Marine Park Authority Research and Monitoring Section June 1990 A REPORT TO THE GREAT BARRI ER REEF MARINE PARK AUTHORITY © Great Barrier Reef Marine Park Authority ISSN 1037-1508 ISBN 0 642 17384 2 Published by GBRMPA June 1992 The opinions expressed in this document are not necessarily those of the Great Barrier Reef Marine Park Authority. Baldwin, C. L. (Claudia L). Impact of elevated nutrients in the Great Barrier Reef. Bibliography. ISBN 0 642 17384 2. 1. Water quality - Queensland - Great Barrier Reef. 2. Waste disposal in the ocean - Environmental aspects - Queensland - Great Barrier Reef. I. Great Barrier Reef Marine Park Authority (Australia). II. Title. (Series: Research publication (Great Barrier Reef Marine Park Authority (Australia») ; no. 20). 333.91709943 Great Barrier Reef • Marine Park Authority PO Box 1379 Townsville Qld 4810 Telephone (077) B1881 1 TABLE OF CONTENTS Executive Summary Section One: Status Report Background and Justification for Action Why Concern with Nutrients Detrimental Effects of Enhanced Nutrients in Tropical Marine Waters Mangrove andSeagrass Environments Molluscs/Crustaceans Case Studies: Nutrients and the AlgaVCoral Relationship Circulation Effects Why Nutrients Impact Coral Communities Changes in the System Coral Calcification Suspended solids,surfactants, and chlorine Nutrients and Crown-of-Thorns Starfish Evidence of Enhanced Nutrients in the GBR Green Island Hayman Island Nutrient Levels in the GBR compared to other Reefal Environments A Relative Perspective of Existing Nutrient Inputs into the GBR Region Point Source Discharges River Input Oceanic Intrusions Groundwater Inputs Minor Inputs Sediment Resuspension Summary: Section One Section Two: Management Implications Introduction Legislation Discharges Outside the Marine Park Discharges Within the Marine Park Cooperation between GBRMPA and Queensland Government Agencies Recommended Management Action Research and Monitoring Requirements Summary: Section Two Section Three: Guidelines for Point Source Discharge into the Marine Park Introduction Factors to Consider in Development of Guidelines Prevention Page 1 3 4 6 6 6 7 7 11 12 12 13 13 14 17 17 19 20 20 21 22 23 23 23 24 26 28 29 29 29 29 31 32 33 34 36 36 37 \I Sewage Treatment Primary Treatment/Septic Secondary Treatment Tertiary Treatment Nutrient Removal: Control of Nitrogen and Phosphorus in Effluent Discharge Sludge Treatment Effluent Disposal Land Disposal of Treated Effluent Ocean Outfall Disposal of Treated Effluent Tolerance Levels Required Dilution Factors Sewage Sludge Disposal Nutrient Removal Requirements Elsewhere Kosciusko National Park Cyprus Sweden Options for Discharge Standards ~~~ 37 37 38 38 39 40 40 40 41 41 42 43 43 43 44 44 45 ~ Option Two Management Recommendations Recommended Guidelines Recommended Information Required in a New Permit Application Recommended Conditions for Sewage Discharge Permits Compulsory Optional Incentives to Tourist Operators Summary: Section Three Tables Table 1 Table 2 Table 3 Table 4 Table 5 Table 6 Table 7 Table 8 Table 9 Table 10 References Personal Communications Appendices Appendix One: Explanation of Tem1inology 46 48 48 48 49 49 49 50 50 Summary of Case Studies GBR Water Quality Summary Comparison of GBR Data with Overseas Data Status of Island Discharges Within the Marine Park Volumes of N & P Loads, Sewage Discharges to North Queensland Waters Comparative P Loads: Riverine Stom1flow, Urban Sewage Discharge, Island Resort Drainage basin area ... total phosphorus load adjacent to Cairns . Section of Marine Park Resulting N & P for Various Levels of Sewage Treatment Required Dilution Ratios for Treated Sewage Sewage Treatment Works Augmentations 65 71 72 1 EXECUTIVE SUMMARY Nutrient levels in inshore GBR waters are reaching levels that have caused detrimental effects to corals elsewhere, though the evidence of damage to coral communities in the Marine Park is still primarily circumstantial. Preliminary studies indicate that nutrient levels in the central GBR are almost twice as high as those in the northern more pristine waters. Whether levels of nutrients have increased in parts of the Marine Park over the past couple of decades has still not been established. Further research is required to evaluate the actual effect on GBR coral reef biota of present levels of nutrients and the levels of nitrogen and phosphoTUs and exposure time required to result in both short and long term damage to coral reef communities. Appropriate research and monitoring to resolve these questions are long term and costly. In the meantime, the implications of Reef deterioration are serious and consideration must now be given to ensuring that levels of nutrients do not increase in the future due to human activities. Sources of nutrient input into the Marine Park are many and range in volume, extent of impact, and continuity. Minor inputs such as shipping and dredging are regulated, not only by the GBRMP Act but also by the Commonwealth Environment Protection (Sea Dumping) Act 1981 and Protection of the Sea Legislation Amendment Act 1986. The latter, being the means of implementation of Annexes IV and V of the MARPOL Convention, has important implications for ports, marinas, and boat construction. Relevant information needs to be directed to those affected. Terrestrial TUn-off is a major source of nutrient input to Reef waters. As the central GBR is more greatly subjected to heavy TUn-off, due to higher rainfall and the reef being close to the coast, management action should focus on this area. Consultation with Queensland government agencies is essential to address this challenge. Point source discharges into the Marine Park may have serious but relatively localised effects. The scale of impact is related to the volume of nitrogen (N) and phosphorus (P) discharged, circulation characteristics of receiving waters and whether the discharge is chronic. Most major coastal urban discharges are to rivers adjacent to the Marine Park and are thus under Queensland jurisdiction. The Marine Park Authority has a clear mandate to regulate discharges directly into the Marine Park, such as discharges from island and coastal resorts and pontoons. These are identifiable and relatively controllable inputs. This paper recommends guidelines for point source waste discharge subject to consultation with appropriate Queensland government agencies. Recommendations 1. A major long term objective is that present levels of nutrients in GBR waters not be allowed to increase through human use. Where existing levels near coral reefs are shown to be higher than those which are compatible with coral reef health or which have occurred historically, the levels should be reduced to levels which are compatible to coral reef health. 2. Attention to direct waste discharge into the Marine Park needs to be given a higher priority by appropriate government agencies and by tourist operations. It is reasonable to expect that, where necessary, upgrading of treatment facilities will be phased in over a period of time to take account of the facility cost, operator training requirements, and to provide time for feedback from monitoring programs. 2 3. Applications for permits to discharge waste into the Marine Park will be considered on a site specific basis, taking into account alternative methods of disposal, proximity and condition of environmentally sensitive sites, hydrodynamics, and ambient water quality. 4. Applicants for new discharges should be required to instal the equivalent of secondary treatment with provision for nutrient removal to be added at a later stage. In environmentally sensitive areas, applicants should be required to establish that the proposed treatment process and dispersion characteristics are such that ambient nutrient levels or levels compatible with reef health at such sites are not increased. If secondary treatment and use of prevention and dilution techniques do not meet established criteria, nutrient removal should be considered. 5. To accurately determine characteristics of effluent from tourist operations, all permittees will be required to monitor nitrogen and phosphorus in effluent on a fortnightly basis at their expense over the next year. Additional monitoring parameters may also be required in consultation with Queensland government agencies. Sampling will be designed to be representative taking into account peak discharges. 6. A thorough assessment of existing treatment plants which discharge into the Marine Park should be undertaken with site visits to inspect treatment plant maintenance, outfall location, and effects on adjacent sensitive sites. 3 SECTION ONE STATUS REPORT 4 BACKGROUND AND JUSTIFICATION FOR ACTION Increasing concern with water quality in the Great Barrier Reef Marine Park and its effects on coral reef communities has been developing for some time. In May 1984 the Great Barrier Reef Marine Park Authority (GBRMPA) sponsored a Workshop on Contaminants in Waters of the Great Barrier Reef Marine Park. The Workshop concentrated on heavy metals, polychlorinated biphenyls (PCBs) and other organochlorines, and hydrocarbons. In attempting to assign priorities to areas of further research, participants noted that sediments and nutrients were more likely to be of greater concern to the Reef than the three contaminant groups considered at that workshop. In particular, an area recommended for further research was: "the effects of agricultural fertilisers and other nutrients exported to the GBR from the mainland" (Dutton, 1985) As a result, in 1987 GBRMPA held a Workshop on Nutrients in the Great Barrier Reef Region. General concern was expressed that inshore waters of the Great Barrier Reef Region appear to have nutrient levels elevated above those likely to be natural and in localised areas may be reaching an undesirable threshold: in the Cairns area (where reefs are close to the coast and the northerly flow of water concentrates nutrients), in the Townsville-Magnetic Island area (where urban sewage discharges may be reaching the inner Great Barrier Reef) and in the Whitsunday area (where there are a number of tourist resorts and intensive tourism activity in a small area with a complex water circulation pattern and high levels of suspended sediments) (Baldwin, 1988). Green Island and Low Isles reefs, two innershelf reefs off Cairns and important tourist sites, may be showing signs of the effects of exposure to water with high nutrients and high turbidity. Green Island Reef is recovering more slowly than expected from crownof-thorns starfish which disappeared from there 5 to 6 years ago, and has experienced a prolific growth of seagrass. Low Isles corals are showing low skeletal density and thus weakening of coral skeletons possibly related to excess phosphate. While the Great Barrier Reef (GBR) situation may not yet be critical, Kaneohe Bay, Hawaii provides a well documented example of destruction of a coral reef from chronic nutrient and sediment stress with occasional acute stresses such as storms. This destruction occurred with nutrient levels of a similar order to those recorded in some inshore waters of the GBR Region. Studies in the GBR Aquarium have also confirmed sensitivity of corals to nutrients. If nutrient and sediment levels increase, coral reefs, particularly those inshore, may be exposed to unacceptable levels of stress over and above natural stresses, resulting from: · nutrients from mainland and resort waste discharges and Mariculture operations (especially of concern where adjacent to fringing reefs, and/or areas of poor water circulation) · developments particularly those involving wetland clearing or dredging · accelerating mainland use (involving clearing and increasing use of agricultural chemicals; mainland runoff influence extends at least 30 km offshore in many areas). Should the Reef become degraded and develop the national and international reputation of no longer having the natural qualities which allracted people to it in the first place, 5 Great Barrier Reef tourism and Australian tourism in general can be expected to suffer. There are many examples throughout the world where such coastal deterioration has occurred: beach degradation of Miami, Florida and Honolulu; water quality and associated benthic deterioration in the Mediterranean, Red Sea, and Caribbean. Inbound tourism is Australia's eighth most important foreign exchange earner ($12 billion). 16% of overseas visitors visit the GBR. The value of tourism in the Great Barrier Reef Region has been increasing in realtenns at the rate of 10% per annum (compared with a world-wide growth of only 2.5%) which gives it an estimated gross output of $240 million per annum in 1988 (based on Driml, 1987b). Other important activities, users and economies may also suffer if degradation of the Reef is allowed to occur. Recreational and commercial fishing yields of the Reef and coastal waters may decline. In 1981/82, this represented a total output (in tenns of gross expenditure) of $42.8 million for the former and total output (in tenns of gross revenue) of $36.3 million for the latter. For comparison, in the same year, total output (in terms of gross revenue) for tourism was $73 million (Driml, 1987a). GBRMPA has identified GBR water quality as a major issue and has initiated a major research and monitoring program: in 1988-8925% ofGBRMPA's research budget was allocated to assessing water quality issues. GBRMPA established a Water Quality Advisory Committee to detennine priorities for an integrated water quality monitoring program in the GBR. Funding is being sought from a variety of sources in order to carry out monitoring on a large scale. An increasing number of developers are required to monitor impacts of their developments on the water quality and biota of the Marine Park. Data required by licence and pennit conditions are available. The Queensland Department of Environment and Conservation is involved in these initiatives. Both the Commonwealth and Queensland Governments have a commitment to the protection of the Great Barrier Reef Region as a World Heritage Area. As part of the World Heritage Area is outside the Great Barrier Reef Marine Park and as major sources of nutrient inputs are outside the Marine Park, cooperation between the Commonwealth and Queensland Governments is essential. The Great Barrier Reef Ministerial Council at its meeting on 26 April 1989 discussed the issue of deteriorating water quality and consequent hannful effects on coral reefs and endorsed the continued cooperation and coordination of research and development of standards by the Authority and Queensland Government agencies and Local Government. It was agreed that there was a mutual desire to protect the Great Barrier Reef in perpetuity and that both Governments will continue to work together towards that commitment. As a step in pursuing the commitment, this paper reviews the status of knowledge on the effects of nutrients on the marine environment, in particular on the Great Barrier Reef. It puts into perspective the main sources of concern so that remedial action may be most appropriately and efficiently directed. Guidelines for point source waste discharge are proposed, not necessarily because point source discharges are the greatest source of concern, but rather that with many new tourist developments and revitalisations underway within the Reef Region, this issue is in urgent need of resolution. 6 WHY CONCERN WITH NUTRIENTS Why are we concerned with nutrients in the GBR Region? (a) (b) (c) Studies have shown the detrimental effects of enhanced nutrients in tropical marine waters. There is evidence of enhanced levels of nutrients in GBR waters. There is some initial evidence that these elevated nutrients may be related to environmental deterioration in the Great Barrier Reef Marine Park. This Section will focus on nutrients, their effects and relative sources of input to the Marine Park. Other components often associated with nutrient discharges, suspended solids, surfactants and chlorine, also can have detrimental environmental effects. These will be addressed briefly. DETRIMENTAL EFFECTS OF ENHANCED NUTRIENTS IN TROPICAL MARINE WATERS Detrimental effects of sewage and in particular, elevated nutrients, on tropical environments have been recognised for some time (Smith, 1977; Kinsey and Davies, 1979; Smith et al 1981). Regions where pollution by sewage, run-off, and even groundwater discharges, of coral reefs or tropical coasts have been documented include the Red Sea (Walker and Ormond, 1982); the Caribbean (Tomascik and Sander, 1985; Rose and Risk, 1985; Lapointe and Connell, 1988); Hawaii (Smith, 1977; Smith et ai, 1981; Maragos et ai, 1985) and Spain (Zoffman et ai, 1989). Mangrove and Seagrass Environments Whereas seagrass and mangrove systems appear to be less susceptible than corals to damage from nutrient enrichment resulting from sewage, significant impacts have been reported. Awareness of their sensitivity is important for health of the Marine Park. The addition of nutrients to mangroves may be beneficial in some instances. For example, increased growth rates of the white mangrove in Florida have been reported (Saenger et ai, 1983). Nevertheless, high organic loading to mangrove systems may cause anoxia and increase the turbidity to levels where the resilience and diversity of these systems is adversely affected. The disposal of excessive organic wastes can lead to defoliation and death of trees or may be deleterious to associated flora and fauna, as occurred in Puerto Rico (Hatcher et ai, 1989; Saenger et ai, 1983). Boto et al (1988) strongly recommended that if waste is to be discharged to a mangrove system, effluents should be subjected to preliminary treatment to reduce the organic mailer content prior to discharge. It is suggested that the ability of mangroves to absorb nutrient inputs will be heavily dependent on the placement, timing, quantity and nature of the effluent. While mangrove trees and soils have a capacity to absorb fairly substantial inputs of inorganic nutrients at least in the short to medium term, their waterways contain very low levels of dissolved nutrients. Direct inputs of nutrients into these waterways could lead to rapid and substantial eutrophication particularly where tidal flushing may be limited (Boto et al 1988). Furthermore, where discharges contain significant amounts of heavy metals or other harmful wastes, toxic bioaccumulation in fish, crustaceans and molluscs, and other residents of these systems, may occur (Saenger, 1989). 7 While seagrass biomass may increase somewhat following mild nutrient enrichment, macroalgae dominate over seagrasses under conditions of marked eutrophication, leading to seagrass death. This effect is due to the growth of epiphytes and associated looselying species (eg Ulva, Ellteromorpha, Ectocarpus) which may originate as attached epiphytes, and which derive most of their nutrients from the water column (Hatcher et aI, 1989). Enhanced growth of epiphytes in nutrient-enriched water was determined to be the cause of large-scale elimination of seagrass meadows in Cockburn Sound, Western Australia and in Port Adelaide. It has also been hypothesised that just as seagrass acts to trap sediment, when it dies silt is more easily resuspended resulting in increased turbidity. Thorhaug (1981) claims that seagrasses have an aesthetic clarifying effect on water quality, by baffling particles from turbid water and keeping sediment bound in place. This of course, is an asset in tourist locations. Molluscs/Crustaceans One of the obvious effects of pollution has been the reduced availability of traditional oyster and clam grounds because of shellfish contamination with bacteria and viruses from domestic sewage. At the larval stage, oysters are extremely sensitive to pollutants such as detergents, pesticides, herbicides, and metals. Sublethal effects such as poor reproductive success has also been noted in adult bivalves. Acute toxic effects on oyster larvae from chloramines has been observed in Virginia waters. Chloramines are formed when chlorine from treated sewage effluents and cooling waters reacts with nitrogenous compounds found in sewage. Chloramines are particularly toxic when mixed with seawater. Increased nitrogen levels from agricultural runoff and sewage effluent lowered oxygen levels, causing shellfish mortality offshore of New Jersey valuing $123 million in 1976 (Leonard, 1989). A recent study by Muir et al (1989) revealed significant mortality of prawns at nitrate concentrations as low as 1 mg/l nitrate. Safe levels of nitrate for prawn larvae were not determined by the study, but it was suggested that it could be considerably lower. Thus toxic levels of nitrate may occur several kilometres from an ocean discharge point. Case Studies: Nutrients and the Algal/Coral Relationship A review of some case studies (Table 1) illustrating nutrient effects on reef environments is useful to gain an understanding of the complexity of impacts from nutrients, particularly on the algal-coral relationship and as warning signs to look for which indicate Reef deterioration. In reviewing these case studies, it is apparent that most obvious or extreme impacts from nutrients have occurred where nutrient input to the system is extremely high, is chronic, and/or water circulation is poor. Applicability to the Great Barrier Reef should be viewed in this context. Enhanced growth and increased biomass of Cladophora, a green alga, now covers large areas of inshore waters of Bermuda although it was not reported 25 years ago. In Harrington Sound, it is reported as a dense mat covering 10 ha of the bottom (Bach and Josselyn, 1978) and averaging 10 cm in depth. It is claimed to be a result of cumulative seepage of N-rich groundwaters coupled with efficient utilisation and recycling of dissolved organo-phosphorous compounds (Lapointe and O'Connell, 1989; Bach and Josselyn, 1979). Concentrations of nitrate, nitrite and reactive phosphorus are usually all below IIlM while ammonia levels were generally less than 3 IlM. Studies also indicated 8 that phosphorus is concentrated in the mat relative to the sUlface water (Bach and Josselyn, 1978). As a result of discharge of untreated sewage to a portion of a Grand Cayman fringing reef, Rose and Risk (1985) found significantly greater dead coral substrate in the vicinity of discharge compared to a control site. It was suggested that the six-fold increase in bacteria biomass in reef waters receiving the effluent was linked to a five-fold increase in sponge (Cliona delitrix, a filter-feeding macroborer) biomass at the polluted site relative to a control site. The elevated density of C. delitrix biomass signified a similar increase in the amount of coral (M. cavemosa) skeleton that had been eroded by this sponge and reduced to silt-sized sediment. Though microbial pollution indicators were acceptable in an area of treated waste discharge near San Gabriel in Alicante, Spain, levels of nutrients were very high, with resulting degradation of local marine ecology and aesthetic values. (Zoffman et ai, 1989). In the increasingly urbanised Florida Keys, Phormidium, the microfilamentous bluegreen alga that causes black-band disease in corals, is becoming chronic on reefs, especially those adjacent to Key West. Those reefs are influenced by the discharge of 8 million gallons per day of raw sewage effluent into upstream surface waters. Black-band disease is particularly well known for its ability to rapidly erode coral cover, which then becomes overgrown by large frondose algae (Lapointe, 1989). A survey by Veron and Kuhlman of reefs around Ishigaki Island, Japan found that nearly all reefs have been damaged or are stressed by human activity. Intensive construction has led to increased siltation of nearby reefs. Heavy use of agricultural chemicals has caused widespread eutrophication and chemicals are having sublethal effects on corals. The survey found that the amount of reef destruction varied according to the source of pollution (Kuhlman, 1988). The stages of deterioration were: in stage one, lower species diversity and coral cover; stage two, white-band disease and other infections; stage three, lower density of the more resistant corals with overgrowth by algae, zoanthids, and sponges, and increased crown-of-thorns starfish. Localised pollution by sewage discharge and phosphate dust from ship loading of coral reef areas at Aqaba, Red Sea contributed to coral death approximately 5 times greater in the polluted area than in the control area (Walker and Ormond, 1982). Growth of algae (Ulva lactlla and Enteromorpha clathrata) was greatly stimulated near the outfall but it appears that algal growth was not the direct cause of coral death. It is suggested that sediment load was increased by the sediment trapping capacity of enhanced algal growth. Phosphate levels in the sewage area were over three times those in the control area posibly reducing calcification of corals. There was no elevation in nitrate and nitrite and no analysis done for ammonia, though increased growth of Ulva as observed is a reliable indicator of elevated ammonia levels. In addition, the density of sea urchins, Diadema setosum, in the sewage area was three times that in the control area. It was concluded that coral was under stress because of the reduced light intensity, inhibition of calcification by excess phosphate and increased sediment load. Archer (1987) reports that Barbados' offshore bank reefs remain healthy whereas the nearshore fringing reefs have been deteriorating since clearing of the virgin forest for cane plantations in the seventeenth century. This resulted in low coral cover by 1977, compared to similar reefs in the Caribbean. Tomascik and Sander (1985) found in Barbados that growth rates of coral subjected to pollution were negatively cOITelated with nitrogen and phosphate. However, they 9 concluded that reduced growth rates of corals at Barbados were a direct result of increased suspended particulate matter (SPM) brought about by increased eutrophication. It was suggested that SPM up to a certain concentration may be an energy source for corals, and that corals use the additional organic fraction of SPM to increase skeletal extension rates. At some point, depending on the coral species, optimum growth will be attained, after which reduction of growth occurs because of the negative effect of decreasing light intensity, physical smothering and reduced zooxanthallae photosynthesis. The study indicated that coral diversity declined and asexual reproduction became more common. In addition, the researchers claim that their data supported the hypothesis that short-term sediment loading or high resuspension rates of short duration do not affect coral growth rates (in terms of skeletal extension) to the same extent as low but persistent sediment loading and/or chronic turbidity. The total phosphorus and inorganic phosphorus concentrations in skeletons of the corals Mon/astrea annu/aris and Dip/oria strigosa from Bermuda, St Croix in the US Virgin Islands and Curacao were shown to be larger in the polluted area than those from relatively pristine sites (Dodge et ai, 1984). Polluted sites were located close to sewage outfalls on all three islands and total phosphate levels were up to twice "control" levels. In the Great Barrier Reef Aquarium in Townsville, elevated nutrient levels have been linked with the death of corals. In 1987, the nitrate concentrations when accelerated coral death occurred in the tank were above 2.5 ~M with phosphorus following closely the pattern of nitrogen. Acroporids appeared to be the most sensitive, with increased death rate ocurring when nitrate concentration was 0.8 ~M. This value is a marked increase over general levels on a coral reef, but is low compared to concentrations that may be expected within the vicinity of a waste water discharge. Further, the nitrate spikes associated with coral death in the tank were short-term events lasting 3 days and higher coral mortality might ensue if elevated nutrients persisted. As the problems appeared to result from release of nutrients from disturbed sediment, the importance of the sedimentary nutrient pool and the danger of suspending sediment in a confined or restricted area must be highlighted (Morrisey, 1988). It should be noted however that the system is totally closed and periods of elevated nutrients might also be coincident with periods of elevations in other undesirable substances (Kinsey, pers.comm.). The most comprehensive case history of sewage effects on reef communities is provided by the studies of Kaneohe Bay, Hawaii. Kaneohe Bay, in particular the poorly flushed southeast sector, was subjected to a chronic stress, receiving increasing amounts of sewage over 30 years. Most of the wastewater received secondary tr.fatment after 1963 and by 1977 the total sewage effluent volume totalled over 2‫סס‬oo m-per day. Most of the sewage was diverted from the Bay to an ocean outfall in 1977 and 1978. The Bay has also been subjected to episodic stresses from stream run-off after heavy rain. A large amount of the community shift occurred since a major surface reef kill in 1965. Kinsey (1988) claimed that by 1977 the Bay community structure indicated a failure to recover from the 1965 kill because of well established chronic stresses. It was speculated that eutrophication and sedimentation as a result of urbanisation and construction, were the major cause of an observed decline in lagoon coral communities in the south lagoon and explosive growth of the green algae Dictyosphaeria cavemosa, which was smothering coral in the middle lagoon. Surveys documented changes to the lagoon before and after diversion of sewage from the lagoon. Some of the most important findings of these studies are summarised as follows. A survey by Maragos in 1972 revealed that compared to earlier studies, 99.9% of the coral reefs in the heavily polluted southeast sector had been eliminated, as were 87% of the corals in the transitional sector and 26% in the nOIthwest sector. The increased levels 10 of nutrients, especially nitrogen and phosphorus, and associated food chain relationships resulted in the following changes in the community structure of the Bay: (I) phytoplankton and zooplankton grazers increased dramatically, especially in the southeast sector. populations of benthic filter-feeders ego sponges and zoanthids increased in response to increased food supply a sediment-feeding sea cucumber appeared in large numbers on organic rich sediments in the southeast sector the growth of benthic algae, especially the "bubble alga" Dictyosphaeria cavernosa was greatly stimulated corals decreased in abundance (Marsalek, 1987). (2) (3) (4) (5) Many of the changes in response to sewage input were reversed slowly after sewage outfall was diverted offshore. Smith et al (1981) monitored the bay ecosystem response by measuring physical, chemical and biological characteristics before and after actual sewage diversion. Initial response of the ecosystem after diversion was quite rapid. Dissolved inorganic and particulate nitrogen, chorophyll and plankton biomass decreased by about 30% resulting in increased water clarity and more favourable conditions for coral growth. Within a few weeks, sponges and zoanthids began to die off in some areas. One year post-diversion, Smith reported that flora and fauna of the bay had not returned to presewage conditions, though there was a dramatic decline in Dictyosphaeria in the middle bay. The sea cucumber was still very abundant. There was little apparent recovery of corals at that time. By 1983, Maragos et al (1985) found a remarkable recovery of corals, especially Porites and MOlllipora sp. Less common coral species showed substantial increase in abundance and distribution throughout the entire lagoon up to 10 Ian away from the site of major impacts. Dictyosphaeria had declined greatly except for a minor increase in the northern lagoon. It is expected that coral will eventually repopulate portions of the bay, although some areas will remain unavailable to coral because of changes in the originally hardbollom substrate now covered with a layer of organic rich sediment (Marszalek, 1987). Maragos (1985) commented on the difficulty in distinguishing between the negative effects of sewage from that of sedimentation since both were concentrated in the south bay during the same time. However, as the dominant species of coral in the bay appeared to be more sensitive to sewage and more resistent to sedimentation, and because sedimentation could be only a minor factor in the decline elsewhere in the lagoon, it is suggested that the rise and fall of the volume of sewage discharged is the best explanation for most of the decline and recent recovery of lagoon corals. Corals introduced to the area also died in direct relationship with their proximity to the sewage discharge point. Kinsey (1988) concludes that reefs may tolerate elevated nutrient levels well above the natural range for significant periods of time with the community structure not superficially reflecting the chronic nutrient stress for a long time. However, elevated nutrients will always result in suppressed community calcification resulting in decreased real growth and structural maintenance. The rate of change will be accelerated dramatically by the occurrence of an acute event, the recovery from which will clearly II reflect adaptation to the chronic stressor. Recovery from such community structure modification can occur within a few years if the chronic stress is removed and if good larval input and suitable substrate are still available. Monitoring the regrowth of coral reef communities following substantial anthropogenic degradation indicates that recovery is typically slow, in the order of years and decades, and often incomplete (Holthus, Evans and Maragos, 1986; Hatcher, Johannes, and Robertson, 1989). This contrasts with quite rapid recovery rates following many natural disturbances (Brown and Howard, 1985; Pastorak and Bilyard, 1985). One explanation is that anthropogenic perturbations tend to be chronic while natural perturbations are infrequent though occasionally severe (Kinsey, 1988; Hatcher et ai, 1989). Circulation Effects At Davies Reef, Furnas et al (1989) found that lagoonal phytoplankton biomass and production were inversely related to wind strength. Production and b.iomass were highest during a mid-summer calm period when water residence times were on the order of several days, but differed little from values measured in surrounding waters during a period of high winds when the residence times were less than one day. Phytoplankton blooms develop within GBR reef lagoons during intennittent calm periods when water residence times exceed phytoplankton generation times. Water residence times can range from a single tidal cycle for a microatoll (Kinsey and Domm 1974) through to several days for a platfonn reef lagoon, to months for the lagoons of oceanic atolls (Furnas et al 1989). Studies done as patt of the Crown-of-Thorns Research Program found that areas of high residence often occur along the northeast or southwest corner of each reef and longer residence times will be experienced by particles which remain close to the sea bed rather than those which reside near the surface (Moran et ai, 1990). Whereas large outfalls in well flushed turbulent open-coast regions appear to have minimal impact on coral reefs (Pastorok and Bilyard, 1985), even small scale discharges, if not effectively flushed, can cause severe problems. The Kaneohe Bay situation indicates that detrimental effects of sewage on corals are generally magnified in confined embayments with restricted circulation (Maragos et al 1985). It is worth noting that the diversion of sewage away from Kanohoe Bay to the ocean outfall has had no noticeable adverse impact on the reef communities adjacent to the outfall. The site is exposed to strong currents, waves and water circulation, with residence times measured on the order of hours, preventing a build-up of nutrients and plankton biomass (Maragos et ai, 1985). Studies in Alicante, Spain also indicated that the shape, structure and orientation of the coastline was a factor in detennining the degree to which beaches were affected by untreated sewage (Zoffman et ai, 1989). Even though enonnous quantities of sewage were discharged along the Miami coast in Florida, effluents were rapidly diluted and dispersed by the adjacent Florida current once the outfall was extended to several kilometres offshore, resulting in a marked reduction in coastal pollution (Marszalek, 1987). This is contrasted with a situation reported by Johannes (1972) where seepage from a single cesspool serving a public restroom in Hanauma Bay brought about the localised degeneration of the nearby coral community. Attached algal populations were found to be larger than normal in this area, with much of the coral dead and encrusted. In summary, then in assessing potential for impact of nutrients on the GBR, it is important to take account of the volume of nutrient input, the degree to which it is a chronic source and the dispersal and dilution characteristics of the receiving waters. 12 WHY NUTRIENTS IMPACT CORAL COMMUNITIES Changes in the System Pastorok and Bilyard (1985) note that coral reef ecosystems are extremely sensitive to environmental perturbations. This high sensitivity is linked to three factors. corals have narrow physiological tolerance ranges for environmental conditions the interactions of key reef species ego algal-coral competition are susceptible to pollutant stresses. Destruction of coral by pollution leads to the eventual demise of many reef species dependent on living coral for food, shelter and refuge from predators. (iii) the effects of toxic substances may be enhanced by the high water temperatures common in coral reef environments. Coral reefs thrive in nutrient poor conditions. Dissolved nutrient concentrations are usually much lower in tropical surface waters than in temperate waters. The elevation of phosphate concentrations by 0.75 JlM in New England waters, for example would result in doubling of phosphate concentration, whereas in the eastern Caribbean it would constitute an approximately 40-fold increase. The possibility exists that the impact of a given increase in nutrient concentrations on a nutrient-poor tropical marine community might be much greater than that on a typical temperate marine community (Hatcher et aI, 1989). Birkeland (1987) postulates that the pattern of nutrient availability is a major determinant of large scale differences in benthic community structure in the coastal environments of the tropics. As illustrated by the previous case studies, long term addition of relatively small amounts of nutrients can cause major imbalances in existing coral reef communities. The growth of mat-forming, attached and planktonic algae is promoted, as are the food webs associated with those algae (Lapointe and O'Connell, 1989). An increase in filter feeders such as sea cucumbers, sponges, and zoanthids (Maragos, 1972; Smith et ai, 1981; Rose and Risk, 1985) and herbivorous fish has been observed. Algae can affect coral by interfering with complex life processes which norn1ally occur at the coral surface, by competition for light and nutrients, by shading and overgrowth (Marszalek, 1987). Breakdown of planktonic algae can add to the sedimentation load. High suspended sediment levels in the water column decrease the amount of light available to corals, reduce zooxanthallae photosynthesis and can lead to eventual physical smothering (Tomascik and Sander, 1985). Increased sediment loads on corals have also been attributed to the sediment trapping capacity of attached algae such as Ulva lactua and Enteromorpha clathrata (Walker and Ormond, 1982). Progressive dominance by soft benthic algae may further decrease suitable hard substrate sites available for coral colonisation (Kinsey and Davies, 1979). An increase in boring sponges and worms can provide an additional threat to coral. Thus a decrease in coral cover, taxonomic richness and net calcification as a result of nutrient enrichment has been reported by many authors (Kinsey and Davies, 1979; Smith et ai, 1981; Walker and Ormond, 1982). A general reduction in numbers of predator fishes may be related to the absence of living corals and reduced habitat complexity (Smith et ai, 1981). Shinn (1989) claims that corals are remarkably resistant to suspended sediments when unaccompanied by the additional stress of excess nutrients or extreme temperature fluctuations. When over-fertilized, rapidly growing blue-green algae, fungi, and bacteria, normally held in check by herbivorous fishes and sea urchins, out compete the corals. (i) (ii) 13 Coral Calcification There is increasing evidence that coral growth and calcification are negatively affected by enhanced phosphorus. Environmental factors which influence calcification in coral are light, temperature, salinity, suspended sediment, nutrient availability and sexual activity (production of gametes diminishes energy available for growth and calcification). Simkiss (1974) claimed that phosphates were crystal poisons of calcification, influencing deposition of calcium in animals with calcareous skeletons. He showed that phosphate inhibits the precipitation of calcium carbonate from artificial seawater at concentrations as low as to 11M (Brown, Ducker, and Rowan, 1977). In relation to coral, Simkiss (1964) suggests that though the role of symbiotic zooxanthellae in the coral tissues in influencing calcification is unknown, their beneficial effects may be related to the removal of phosphates as inhibitors of calcification (Brown and Scoffin, 1986). In fact, in contrast to the response to nitrogen, several biochemical characteristics suggest that zooxanthellae freshly isolated from corals have high levels of the phosphate uptake system and levels of phosphatase that are typical of the P-starved algae (Yellowlees et ai, 1988). Kinsey and Domm (1974) tested the effects of discontinuous fertilisation of a lagoon patch reef system at One Tree Island, Great Barrier Reef. Enough phosphate was added to maintain 2 11M during a three hour period, an increase of to-fold over that normally found in the area. N was added to maintain 20 11M urea and ammonium, compared to normal N in the area of less than .5 11M nitrate. The results of Kinsey and Davies (1979) revealed a pronounced increase of about 50% in the rate of net community photosynthesis over that for any equivalent period of the preceding year. The increase was attributed solely to increased production by benthic algae, as tidal washout prevented any appreciable buildup of phytoplankton. A greater than 50% suppression of reef calcification was found in the fertilised area, compared to control corals in the unfertilised areas, attributable to the phosphate (Kinsey and Davies, 1979). The authors also commented that the highest phosphate level reported in the Pacific, 0.6 11M, at Canton Atoll (as per Smith and Jokiel, 1975) was associated with the lowest overall lagoonal calcification rate (as per Smith and Kinsey, 1976). Brown et al (1977) found that the growth of articulated coralline algae is signficantly inhibited by a medium enriched with orthophosphate (30 umol/l) at a concentration normally used in culturing other groups of marine algae. When concentrations of 7.5 and 3.8 11M were used, significant increases in survival and growth were found in coralline algal cultures. Coralline algae are widely distributed from tropical to polar seas. Suspended solids, surfactants, and chlorine Frequently suspended solids, surfactants, and chlorine are found in association with nutrients. Though data is limited on the effects of these water quality parameters on coral reef ecosystems, the possibility of a confounding effect on the environment must not be ignored. Suspended particles in waters of the Great Barrier Reef consist partly of fine inorganic sediments entrained in the water column by turbulence, and partly of particles of organic origin such as detritus, phytoplankton and micro-zooplankton (Bell et al 1987b). Sources can be teITestrial run-off, dredging, storms, and sewage. Sedimentation itself has negative effects on coral by: increasing turbidity, thus reducing photosynthesis; resting on the polyp surface which causes stress through sediment rejection mechanisms such as 14 mucous generation; inhibiting population recruitment; and smothering corals (Pastorak and Bilyard, 1985). Though sedimentation effects on the reef ecosystem needs to be given separate attention, this paper briefly addresses sediments only as they relate to nutrients. Suspended solids in r~ceiving waters for sewage discharges originate from three sources: particles contained in effluents, particulate organic matter produced by nutrient enrichment, and natural seston. The relative importance of these depends on wastewater treatment levels (Bell et al 1987b). McConchie (1988) discusses transport of nutrients through adsorption onto colloidal particles (in this case, primarily fine particles of clays and iron-oxides in water) and subsequent desorption in response to changes in environmental conditions. Since phosphorus adheres to clay particles, increased erosion from agricultural areas where chemical fertilisers are used, can contribute to the nutrient load, though this is likely to be mainly restricted to the nearshore zone. Detergents are actually mixtures containing surfactants plus other substances called builders that enhance the cleansing action (such as sodium tripolyphosphate), bleaching agents, fluorescers, etc. (Bell et aI, 1987b). Evidence of the presence of surfactants is often observed as foam or scum around outfalls. Surfactants (surface active agents) present in detergents as well as in dispersants can have deleterious effects on marine systems, particularly fish, crustaceans, and corals. Chlorine is commonly used as a disinfectant for sewage water and an anti-fouling agent for power-generating and desalination plant cooling water systems. The effect of free residual chlorine on many marine organisms is unclear. Unchlorinated domestic sewage has been found to be a relatively weak inhibitor of external fertilisation in marine invertebrates, but chlorinated sewage was a potent spermicide, active in inhibiting fertilisation at levels as low as 0.05 ppm (Bell et ai, 1987b, as per Muchmore et ai, 1973). Evidence of the effect of chlorine on coral colonies comes from the Bahamas where chlorine bleach used to hunt fish has inadvertently spilled on coral causing infection and coral mortality (Bell et ai, 1987b). Nutrients and Crown-of-thorns Starfish (COT or Acallihasier plallci) Some of the previous case studies have described a relationship between enhanced nutrient levels and certain invertebrates, such as seacucumbers and echinoderms, in particular D/adema sp. Though no such direct relationship has been found between nutrient levels and crown-of-thoms starfish, one of the many hypotheses concerning the causes of A. plallei outbreaks relates increased terrestrial runoff and possibly enhanced nutrients to increased survival of A. pia/lei larvae. Neither the "larval recruitment" hypothesis or the "terrestrial run-off' hypothesis have been totally accepted or rejected. Limitations on current knowledge of the population biology of A. plallei and related areas require that conclusions must await further research. In all likelihood, COT outbreaks are a result of a combination of contributing factors, both natural and human induced. The following is a brief synthesis of theories related to elevated nutrients and A. plallc/. For a more detailed discussion and critical appraisal of the above-mentioned hypotheses and others, it is suggested that the reader refer to Moran (1988). These hypotheses are discussed here as they have provided some of the incentive for research into nutrients on the GBR, and because future research and management action regarding nutrients should not disregard the potential implications of these hypotheses. Both the "Larval Recruitment Hypotheses" and "Terrestrial Run-off Hypotheses" are based on the postulation by Birkeland (1982) that large fluctuations in the abundance of 15 A. planci are the result of differential survival of larvae rather than of any other stage of the life cycle. He argued that outbreaks arise from periods of successful recruitment, not from a decrease of predator pressure which would result in the gradual build-up of individuals over a number of years. Examination of existing data on A. planci strongly supports the idea that population outbreaks result from years of high recruitment success (Olson, 1987). Lucas (1975) suggested the following factors may be important in affecting the survival of larvae and early juvenile stages: degree of fertilisation, abundance of food, temperature, salinity, extent of predation, dispersal and availability of suitable substrata for settlement. The Larval Recruitment hypothesis proposes that recruitment of larvae of A. planci is enhanced during times of favourable environmental conditions (Moran, 1988). This was based on laboratory studies by Lucas (1973, 1975) which indicated that the survival of larvae is improve.