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Copepod distributional ecology in a glacial run-off fjord Stone, David Philip

Abstract

The Pacific coast of Canada is indented by numerous fjords. However, there has been no synoptic zooplankton study of a fjord in British Columbia, and little information is available to suggest how spatial and temporal distributions may change along a fjord's length in response to variation in hydrographic circulation, "water quality", and distribution of phytoplankton. The investigation reported here was designed to help fill this gap. The study area was Knight Inlet, a local glacial run-off fjord, partitioned by a sill into a shallow outer (200 m) and deep inner (500 m) basin. Ten cruises were made to the area between October 1974 and September 1975. Vertically discrete zooplankton hauls were taken over a standard depth range of approximately 16 km intervals along a transect from Queen Charlotte Strait to the fjord head. All observed calanoid copepods (the group which dominated the zooplankton) were counted at the species sexed copepodite level. Salinity, temperature, oxygen, nitrate, chlorophyll a, and suspended sediment data were collected concurrently and plotted as isopleth profiles, from which hydrographic circulation was deduced. The profiles, in combination with Temperature-Salinity diagrams, were also used to "partition" the fjord into geographically and vertically discrete "water regimes", each identifiable by a unique suite of conservative and non-conservative properties. All regimes were grouped into either a "Surface", "Transition" or "Deep" category. Dominant features of hydrographic circulation were the summer surface outflow of low salinity glacial run-off, and the replacement of deep waters by a high salinity intrusion associated, with upwelling. New intrusions resulted in up-inlet movement of previously resident waters, which were then uplifted and flushed down-inlet. This counter-current system of flows appeared to act as a nutrient trap, retaining within the inlet any biologically utilisable material, and leading to the accumulation of high nitrate concentrations in the inner basin. Monthly Temperature-Salinity-Plankton (T-S-P) diagrams showed that five copepod species groups could be recognised according to an apparent association with either one or two water regimes. They were named accordingly, "Summer Surface", "Surface and Surface Transitional", "Transitional/Deep", "Deep", and "Off-shore". A final group was designated "Migrant", and contained all diel and seasonal vertical migrants. Monthly profiles of species presence/absence, and profiles of conservative and non-conservative properties provided a spatial aspect to water regime-plankton associations revealed on the T-S-P diagrams. For example, most Transitional/Deep and all Deep species were clearly associated with the inner basin, whilst most Surface and Surface/ Transitional species appeared to be associated with the outer basin and Queen Charlotte Strait. This procedure also revealed the advection of groups into "unusual" locations or depth ranges. For example, when deep inner basin water regimes were uplifted, similar upward displacement of Deep species were observed. Similarly, copepods characteristic of an off-shore fauna were carried into Queen Charlotte Strait by the July intrusion, and small numbers were advected into the fjord outer basin. The breeding cycles of herbivorous copepods varied within species at different geographical localities. This appeared to reflect the almost complete disappearance of phytoplankton from the inner basin after the arrival of turbid glacial run-off into the fjord head from June until September. Deep species showed little seasonality in breeding cycle and a trend towards this situation was observed in the Transitional/ Deep group. In conclusion, this thesis describes temporal and spatial patterns of distribution for all ealanoid copepod species found in Knight Inlet, and attempts to relate these to fjord hydrography and the distributions of certain environmental properties.

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