In the highly dynamic environment of an estuary, environmental factors such as salinity, temperature, light and hydrodynamics as well as biogenic factors such as predator/prey interactions influence the population dynamics of estuarine species. The aim of my study was to explore the potential for resource partitioning between four commonly co-existing estuarine mysid species, Tenagomysis chiltoni, Tenagomysis novae-zealandiae, Tenagomysis macropsis, and Gastrosaccus australis, by investigating their life histories, seasonalities and distributions within an estuary. Mysid samples were collected along the shoreline (water depth: 0.5 – 0.6 m) of the Taieri Estuary (South Island, New Zealand) using a sweep net and in the deeper water (water depth: 4 – 6 m) with a set of 3 drift nets. A lateral segregation in the estuary was found between the two hyperbenthic species, T. chiltoni and T. novae-zealandiae, and the two benthopelagic species, G. australis and T. macropsis with hyperbenthic mysids mainly occurring in the shallow waters along the estuary shoreline and benthopelagic species in deeper central-channel waters. The life cycle and breeding dynamics of all four mysid species were similar to those described in the literature for other temperate estuarine mysid species, although the breeding period in the study estuary was noticeably shorter than in estuaries of lower latitudes in New Zealand and elsewhere. The observed seasonal migrations and life-stage specific segregation between adults and juveniles may reduce competition, inter-specific predation, and cannibalism by allowing different life stages to utilise the same habitats or food sources, but at different times. Water temperature seems to play a key role for the timing of the breeding season, abundance, and reproductive effort, whereas salinity, total suspended solids and phytoplankton biomass (measured as chlorophyll a) were associated with the distribution of the mysid species. Thus, the spatial distribution of these four mysid species within the Taieri Estuary reflects the complex linkages and interactions between favourable temperature and salinity conditions, food availability and reproduction. As a consequence of confirming the lateral and longitudinal segregation of species and life history stages within the estuary, the question arose of how relatively small aquatic organisms, such as mysids, orient themselves in a large three-dimensional body of moving water. Therefore the role of turbulence as an orientation aid was investigated in a flume tank experiment, using Gastrosaccus australis as the test organism. The mysids actively tracked areas of shear, when turbulence was present, but favoured reduced flow in the absence of turbulence. These different swimming responses might be an adaptation of G. australis to its pelagic habitat. By moving to reduced flow areas in an absence of mid-water turbulence, mysids could avoid seaward displacement.Change in riverine management such as by water abstraction, damming for hydropower plants, or extreme weather events associated with climate change will impact distribution, densities and population dynamics of the resident mysid species in estuaries. These changes in abundance or, in the worst case scenario, even the loss of mysid species could potentially have severe consequences for estuarine food webs, which may result in decreased food availability and therefore impact the nursery function of estuaries for commercial fish species.
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Life history and distribution of mysid species in a large open estuary
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