Marine and Freshwater Research

Sea temperatures in many tropical regions have increased by almost 1°C over the past 100 years, and are currently increasing at ~1–2°C per century. Coral bleaching occurs when the thermal tolerance of corals and their photosynthetic symbionts (zooxanthellae) is exceeded. Mass coral bleaching has occurred in association with episodes of elevated sea temperatures over the past 20 years and involves the loss of the zooxanthellae following chronic photoinhibition. Mass bleaching has resulted in significant losses of live coral in many parts of the world. This paper considers the biochemical, physiological and ecological perspectives of coral bleaching. It also uses the outputs of four runs from three models of global climate change which simulate changes in sea temperature and hence how the frequency and intensity of bleaching events will change over the next 100 years. The results suggest that the thermal tolerances of reef-building corals are likely to be exceeded every year within the next few decades. Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years. Most information suggests that the capacity for acclimation by corals has already been exceeded, and that adaptation will be too slow to avert a decline in the quality of the world’s reefs. The rapidity of the changes that are predicted indicates a major problem for tropical marine ecosystems and suggests that unrestrained warming cannot occur without the loss and degradation of coral reefs on a global scale.

Seasonal change in temperature has a profound effect on reproduction in fish. Increasing temperatures cue reproductive development in spring-spawning species, and falling temperatures stimulate reproduction in autumn-spawners. Elevated temperatures truncate spring spawning, and delay autumn spawning. Temperature increases will affect reproduction, but the nature of these effects will depend on the period and amplitude of the increase and range from phase-shifting of spawning to complete inhibition of reproduction. This latter effect will be most marked in species that are constrained in their capacity to shift geographic range. Studies from a range of taxa, habitats and temperature ranges all show inhibitory effects of elevated temperature albeit about different environmental set points. The effects are generated through the endocrine system, particularly through the inhibition of ovarian oestrogen production. Larval fishes are usually more sensitive than adults to environmental fluctuations, and might be especially vulnerable to climate change. In addition to direct effects on embryonic duration and egg survival, temperature also influences size at hatching, developmental rate, pelagic larval duration and survival. A companion effect of marine climate change is ocean acidification, which may pose a significant threat through its capacity to alter larval behaviour and impair sensory capabilities. This in turn impacts on population replenishment and connectivity patterns of marine fishes.

For a majority of aquatic ecosystems, respiration (R) exceeds autochthonous gross primary production (GPP). These systems have negative net ecosystem production ([NEP]=[GPP]–R) and ratios of [GPP]/R of <1. This net heterotrophy can be sustained only if aquatic respiration is subsidized by organic inputs from the catchment. Such subsidies imply that organic materials that escaped decomposition in the terrestrial environment must become susceptible to decomposition in the linked aquatic environment. Using a moderate-sized catchment in North America, the Hudson River (catchment area 33500 km2), evidence is presented for the magnitude of net heterotrophy. All approaches (CO2 gas flux; O2 gas flux; budget and gradient of dissolved organic C; and the summed components of primary production and respiration within the ecosystem) indicate that system respiration exceeds gross primary production by ~200 g C m-2 year-1. Highly 14C-depleted C of ancient terrestrial origin (1000–5000 years old) may be an important source of labile organic matter to this riverine system and support this excess respiration. The mechanisms by which organic matter is preserved for centuries to millennia in terrestrial soils and decomposed in a matter of weeks in a river connect modern riverine metabolism to historical terrestrial conditions.

Hypotheses to explain the source of the 1011 tons of salt in groundwaters of the Murray Basin, south-eastern Australia, are evaluated; these are (a) mixing with original sea water, (b) dissolution of salt deposits, (c) weathering of aquifer minerals and (d) acquisition of solutes via rainfall. The total salinity and chemistry of many groundwater samples are similar to sea-water composition. However, their stable isotopic compositions (δ18O= –6.5 ‰; δ2H = –35) are typical of mean winter rainfall, indicating that all the original sea water has been flushed out of the aquifer. Br/Cl mass ratios are approximately the same as sea water (3.57 x 10-3) indicating that NaCl evaporites (which have Br/Cl<10-4) are not a significant contributor to Cl in the groundwater. Similarly, very low abundances of Cl in aquifer minerals preclude rock weathering as a significant source of Cl. About 1.5 million tons of new salt is deposited in the Murray–Darling Basin each year by rainfall.The groundwater chemistry has evolved by a combination of atmospheric fallout of marine and continentally derived solutes and removal of water by evapo-transpiration over tens of thousands of years of relative aridity. Carbonate dissolution/precipitation, cation exchange and reconstitution of secondary clay minerals in the aquifers results in a groundwater chemistry that retains a ‘sea-water-like’ character.

Otoliths are of interest to investigators from several disciplines including systematics, auditory neuroscience, and fisheries. However, there is often very little sharing of information or ideas about otoliths across disciplines despite similarities in the questions raised by different groups of investigators. A major purpose of this paper is to present otolith-related questions common to all disciplines and then demonstrate that the issues are not only similar but also that more frequent interactions would be mutually beneficial. Because otoliths evolved as part of the inner ear to serve the senses of balance and hearing, we first discuss the basic structure of the ear. We then raise several questions that deal with the structure and patterns of otolith morphology and how changes in otoliths with fish age affect hearing and balance. More specifically, we ask about the significance of otolith size and how this might affect ear function; the growth of otoliths and how hearing and balance may or may not change with growth; the significance of different otolith shapes with respect to ear function; the functional significance of otoliths that do not contact the complete sensory epithelium; and why teleost fishes have otoliths and not the otoconia found in virtually all other extant vertebrates.

This paper reviews research on fluxes of carbon in Australian floodplain rivers. Except where cover is absent, and in-stream gross primary production is >1 gC m–2 day–1 and ratios of production to respiration are >1, riparian sources dominate carbon pools in catchment streams. On floodplains, primary production by river red gum (Eucalyptus camaldulensis) forests is ~600 gC m–2 year–1. Total primary production by aquatic macrophytes and biofilms in floodplain wetlands is >2500 gC m–2 year–1 and >620 gC m–2 year–1, respectively. Large pools of particulate organic carbon (POC) exist on floodplains as litter (>500 gC m–2) and coarse woody debris (~6 kgC m–2). Floods may release 50 gDOC m–2 from leaf litter. Export of this DOC (dissolved organic carbon) may be substantial relative to autochthonous production in river channels. Sediments deposited on floodplains during large floods represent a substantial sink of riverine POC (up to 280 gC m–2). Bacteria are responsible for rapid decomposition of DOC and POC in floodplain wetlands (sediment respiration and methanogenesis, both ~1 gC m–2 day–1). Flow and its interaction with geomorphology control carbon fluxes in rivers. Decreased inputs of floodplain carbon, following river regulation and physical disturbances to catchments and floodplains, may have resulted in many Australian rivers being dominated by algal production.

Five species of abalone occur along the southern Australian coastline; of these three species, Haliotis laevigata Donovan, Haliotis roei Gray, and Haliotis ruber Leach, are of commercial importance; the other two species are Haliotis cyclobates Peron and Haliotis scalaris Leach. The habitat, movement, feeding behaviour, food, and ecological relationships with predators were studied for each species at three study sites. Each species of abalone occupies a distinctive microhabitat. H. cyclobates lives in calm-water places associated with communities of the seagrass Posidonia australis and the razor shell Pinna dolobrata; H. laevigata lives on open rock adjacent to sand in moderate to rough water localities; H. ruber prefers caves in calm- to rough-water localities; H. roei occurs in narrow crevices in the upper sublittoral on rough-water coasts ; H. scalaris is an under-boulder or crevice-living species. All species are sedentary, but may make local movements in search of food. Several species may occur in a given habitat but there is little microhabitat overlap. The seasonal variation in food eaten by each species is described. All species show preference for red algae and reject most species of brown algae, subsisting predominantly on red algae and seagrasses according to the possibilities of the habitat. H. laevigata feeds mainly on algal drift and H. roei is substantially a grazing species. The other species feed on algal drift or graze opportunistically. Water movement is an important environmental factor affecting the feeding of those species which feed on algal drift. H. laevigata and H. ruber feed best in conditions of moderate water movement but poorly if the water is too calm or too rough. Water movement elicits a characteristic feeding response in these species. The predators of abalone include fish, crabs, molluscs, and starfish; their interaction with abalone is discussed. Crevices, caves, and cavities under boulders provide a refuge in space from predators for H. roei, H. ruber, and H. scalaris and juveniles of other species, which appear to be confined to these places, except for nocturnal feeding excursions, by the activity of their predators. The adults of the different species differ from each other in their dependence on water movement, their preference for open or cryptic sites such as crevices or caves for resting, their size and their methods of feeding. These differences between the species, taken together, ensure that there will be very little overlap between them in the sorts of places that they seek to live in or their behaviour in seeking food; in those cases where food-seeking behaviour is similar, interspecific competition would seem to be negligible because food is abundant. Predation would seem to have been more important than interspecific competition as the selective pressure that established and maintains these differences between the species.

A mathematical model is proposed for the movement of water and sediments in Coral Creek. a tidal creek surrounded by thickly vegetated mangrove wamps in Missionary Bay at the northern end of Hinchinbrook Island, northern Queensland. It is shown that a net down-stream longitudinal current through mangrove swamps exists and is responsible for rapidly carrying away organic detritus (e.g. fallen tree leaves) to the ocean, thereby enhancing the export of nutrients from mangrove swamps. The asymmetry between ebb and flood currents contributes to the maintenance of a deep self-sustaining drainage channel. even in the presence of a large sediment input from surrounding coastal waters. The size and geometry of the drainage channel are related to vegetation density in mangrove swamps. Evapotranspiration from mangroves in Coral Creek is responsible for the existence of a longitudinal salinity gradient. The evapotranspiration rate could be as large as 3 cm,'day. Residual currents are clockwise in Missionary Bay and explain the overall patterns of scour and deposition in the bay.

Estuaries are especially vulnerable to the impacts of climate change because changes in climatic and hydrologic variables that influence freshwater and marine systems will also affect estuaries. We review potential impacts of climate change on Australian estuaries and their fish. Geographic differences are likely because southern Australian climates are predicted to become warmer and drier, whereas northern regions may see increased precipitation. Environmental factors, including salinity gradients, suspended sediment, dissolved oxygen and nutrient concentrations, will be influenced by changing freshwater input and other climate variables. Potential impacts will vary depending on the geomorphology of the estuary and the level of build-up of sand bars across estuarine entrances. Changes to estuarine fish assemblages will depend on associated changes to salinity and estuarine-mouth morphology. Marine migrants may be severely affected by closure of estuarine mouths, depending on whether species ‘must’ use estuarine habitat and the level of migratory v. resident individuals. Depending on how fish in coastal waters locate estuaries, there may be reduced cues associated with estuarine mouths, particularly in southern Australia, potentially influencing abundance. In summary, climate change is expected to have major consequences for Australian estuaries and associated fish, although the nature of impacts will show significant regional variation.

A review of the literature suggests that river discharge plumes strongly influence fish larvae and may play a significant role in the recruitment of local fishes. Some rivers drain large land masses to discharge shallow, turbid and nutrient-rich plumes that interact with ocean currents as well as with local oceanography and meteorology; these plumes may extend hundreds of kilometres offshore and alongshore. The frontal, or mixing, zone between plume and ocean waters is characterized by strong physical and biological processes. Physical dynamics, e.g. hydrodynamic convergence, and abundant nutrients (both river derived and upwelled) in the vicinity of discharge plumes often generate large stocks of phytoplankton, zooplankton and fish larvae, as well as high rates of primary and secondary production. Physical dynamics not only act to accumulate (and probably retain) biomass in frontal waters, but also transport organisms onshore, offshore and along the frontal boundary. The mechanisms through which river plumes may influence recruitment are not clear. In considering the potential effects of scale of river discharge on recruitment, three alternative hypotheses are discussed. The short-food-chain hypothesis states that recruitment will be enhanced in the vicinity of river plumes because fish larvae experience superior feeding conditions, grow faster and thus experience a shorter stage duration and survive better. The total-larval-production hypothesis is that trophic conditions support such high total production of fish larvae that specific dynamics of growth and mortality are not relevant. The third hypothesis is that plumes facilitate the retention of fish larvae within a limited area, and it is the physical retention rather than production that explains the variation in recruitment. If one or a combination of these hypotheses explains the influence of river plumes on recruitment, then the greatest potential to affect fish recruitment may be possessed by large mid-latitude rivers carrying high suspended-sediment and nutrient loads that discharge into shelf waters to create well defined plumes where primary and secondary production are high.