The Effects of Floods on Estuarine Fisheries and Food Webs

Abstract

Floods are extreme events that can rapidly alter water and habitat quality in receiving estuaries. Because floods are unpredictable, they are more difficult to study, so have received less research attention than freshwater flow studies, resulting in a paucity of information on their ecological effects in the coastal zone. Previous studies have shown correlations between high flow periods and increased fisheries catches, which suggests that floods stimulate productivity in receiving waters. However, there have been no studies providing direct links between floods and increased productivity responses in fisheries species. In addition, the long-term effects of deposited flood sediment on food webs in estuaries are poorly understood. Floodwaters can carry high loads of fine sediment, which settles at the most offshore portion of the estuary delta, known as a prodelta. Nutrients, trace elements and other substances are also exported from the catchment dissolved in floodwater or attached to fine sediment particles and are deposited in estuaries. However, the processes of nutrient release from suspended sediments and settled sediments, and uptake of nutrients and trace elements into the food web in receiving estuaries are not well understood. Therefore, this thesis used laboratory experiments (Chapter 2 & 3) to study these processes with the aim of gaining a better understanding of the mechanisms underpinning measured ecological flood responses using field studies (Chapter 4 & 5). This study used catchment soils formed from three distinct rock types (granite, basalt and sandstone) from the Brisbane River (Queensland, Australia) catchment in a flood simulation experiment to quantify the rates of nutrient release during flooding (Chapter 2). In the laboratory, the fine fraction (<63 µm) of the soils was tumbled in freshwater for three days and left to settle in seawater for four weeks, and filtered water samples were taken throughout. A thin layer of the fine soils was also added to incubated sediment cores collected from central Moreton Bay, Queensland, Australia, to measure the influx or efflux of dissolved nutrients from sediments. Basalt soils, in particular, were relatively nutrient-rich and released substantial quantities of organic and inorganic dissolved nutrients, particularly phosphate. However, when soils were added to estuarine sediment cores and incubated, there was a net influx of phosphate from the overlying water. All soils continually released ammonium in both experiments, indicating that catchment soils may be an important source of ammonium to fuel productivity within the coastal zone. Catchments can also contribute trace elements to estuaries and coastal areas. Sources of trace element (cadmium (Cd), manganese (Mn) and zinc (Zn)) accumulation in estuarine fisheries species occupying different trophic levels were determined using radioisotope experiments (Chapter 3). Clams, prawns and fish common to estuaries on the East Australian coastline were exposed to a combination of Cd, Mn and Zn in three sources: dissolved in seawater, attached to suspended sediment particles, and diet. The study showed that clams accumulated all three elements from seawater, while prawns and fish showed negligible uptake. This indicates that clams may be an important link between dissolved and bioavailable elements for higher trophic level species, since the clams bioconcentrated these elements from seawater. The suspended sediment exposures had similar outcomes, with accumulation by clams, with negligible uptake by prawns and fish. Clams may therefore be particularly sensitive to accumulation of contaminants during floods, which may be exacerbated by their sessile nature. Biokinetic modelling using moderate environmental metal concentrations showed that diet was the main source of Cd and Zn accumulation in clams, whereas seawater was likely to be the main source for clam Mn accumulation. Diet was found to be the main source of Cd, Mn and Zn accumulation in prawns and fish, which is supported by previous studies. In the diet exposures, there was also a considerable difference in assimilation efficiencies between prawns and fish. The results of this study suggest that the contribution of diet to trace element accumulation at higher tropic levels may be larger than previously thought, and diet should therefore be considered in trace element studies involving high trophic level species. To identify short-term responses to a flood, multiple condition indices were determined for brown tiger prawns collected before and after a large cyclone-driven flood from several sites in central Moreton Bay (Chapter 4). Prawns collected from sites closest to the Brisbane River and Moreton Island showed no consistent change in condition over time. However, prawns collected from the most southern sites closest to the Logan River, which was the most severely flooded river system during the storm, responded positively to the flood with increased condition measured in terms of length-weight relationships, carbon-nitrogen ratios and muscle lipid content. Peak condition was measured on the first post-flood sampling occasion (i.e. day 11), and prawns collected 53 d after the flood were found to be in a similar condition to those before the flood. This peak in prawn condition coincided with an increase in benthic algal biomass that occurred simultaneously, measured as chlorophyll-a concentrations. Unlike previous studies, which found negative flood condition responses in fish in urban estuaries, the results from this study show that floods in South East Queensland can increase food web productivity in Moreton Bay. Water conditions in estuaries following floods usually return to normal within a couple of months, therefore long-term flood effects are likely to be caused by catchment sediment that is deposited on the estuary prodelta. To assess catchment influences on energy flow to fish and prawns living on flood prodeltas, common fish and prawn species were analyzed for stable isotope and trace element composition (Chapter 5). Fish and prawns were collected from a prodelta in an urbanized catchment (Moreton Bay) and three non-urbanized catchments (Gulf of Carpentaria). Links between diet and trace element composition were found. Differences in trace element composition were greatest between fish and prawns, which was reflected in correlations of trace elements with nitrogen isotopes. Differences within fish and prawn groups were also found, which were correlated to carbon isotopes and reflected benthic versus pelagic diets of different species. These correlations also highlight the importance of diet in trace element accumulation, and show that trace elements can be useful in food web studies. Variations in diet between locations were measured for some species, particularly catfish, suggesting that material exported from the catchment during floods influences food webs through availability of quality prey in estuary prodeltas. Catfish in Moreton Bay were found to be feeding at a high trophic level, suggesting that the urbanization of the Brisbane River catchment does not negatively affect the quality of food available in the Brisbane River prodelta. Overall, the results of this thesis showed that floods, and the sediments they transport to estuaries, can be beneficial for fisheries and food webs in receiving estuaries. In addition, trace element accumulation is linked to diet, and stable isotopes and trace elements can be combined to study catchment influences on estuarine food webs. An effective and transferrable methodology for mapping catchment influences and evaluating food quality in important fisheries habitats was identified. Floods facilitate a seaward transport of water, sediment and substances from catchments that are taken up into food webs in the coastal zone. Flood frequency and intensity is expected to increase with a changing climate and the work presented here fills important knowledge gaps on ecological responses to floods. While organisms have responded to and recovered from flood events throughout history, changes to sediment and water volumes through land clearing and damming of rivers may affect the quality of food, and therefore fisheries productivity, in the coastal zone. This study therefore has important implications for management of land use, erosion and water resource allocation within catchments.