AbstractsBiology & Animal Science

Fiordland, New Zealand, provides a great opportunity to study a near-pristine estuarine fjord ecosystem to study mechanisms that drive the fate of phytoplankton from the water column to the sediment across strong estuarine gradients. Large amounts of freshwater enter Doubtful Sound, creating a highly stratified (physical, chemical) upper water column in the inner fjord, and well mixed water column in the outer fjord. Steep fjord walls and chromophoric dissolved organic matter-rich freshwater, which influences phytoplankton production and degradation, dominate the light environment in Fiordland. In this study, phytoplankton pigments and environmental observations were used to investigate how phytoplankton detritus is degraded, altered, and preserved within the water column and into the sediment record. In particular, degradation and preservation processes within the water column, sediment surface, and in long-term sediment records were evaluated using water samples, grazing experiments, sediment traps, early diagenesis experiments, and sediment cores. The most important factors influencing phytoplankton pigment composition in the upper water column were photosynthetically active radiance, direct irradiance, rain and wind stress, which together together with salinity and temperature explained 86.7% of the variance in the phytoplankton community data. The best explanatory variable for each season investigated were physical parameters in spring and summer (up to 58.4% explained variance) and climate parameters in autumn and winter (up to 48.0% explained variance). Copepod herbivory was a major driver of chlorophyll a destruction and conversion to steryl and carotenol chlorin esters. Diatoms and phototrophic ciliates were the preferred ingested taxa. Grazing removed up to 13% of the daily phytoplankton standing stock. The flux of phytoplankton pigments through the water column generally corresponded with phytoplankton bloom periods. Increased flux of grazing biomarkers coincided with increased flux of chlorophyll a, highlighting the importance of zooplankton fecal pellets as a means of transport to the sediment. Incorporation of phytoplankton detritus into the surface sediment during experiments was enhanced in anoxic versus oxic sediments and the effect of bioturbation was significant (p < 0.05) with time (1 month) within anoxic sediments, increasing detrital burial. The main means of transport for phytoplankton detritus into deep basins in Fiordland is zooplankton fecal pellets as indicated by the large amount of refractory material observed in the long-term record. Furthermore, preservation of phytoplankton pigments and therefore organic matter in the Fiordland basins is good, as highlighted by little variation of chlorophyll a:pheopigment ratios as well as the exponential decay in 210Pb profiles. This study demonstrates the complexity of drivers influencing phytoplankton dynamics, the importance of zooplankton grazing and transport of phytoplankton-derived organic carbon within fecal pellets to the basin floor. In…