The effect of hydrography onthe planktonic community structure and secondary production of an area offthewest coast of the Isle ofMan was investigated in 1993. This region is subject to stratification in summer (May to September), the boundary between mixed and stratified water masses being marked bytheManx West Coast front (MWC). Acoastal front was also occasionally present, separating the mixed water mass from coastal water. Large interannual variability inthe strength of the stratification was apparent between 1993 and 1994. There were large seasonal changes in nutrient concentrations, with maximal concentrations inthe winter months. Nutrient depletion of theupper layer of thewater column in summer was apparent. Phytoplankton spatial distribution was strongly related to the physical structure of the area and higher fluorescence was measured during thegreater stratification in 1994. Abundances were initially highest inthe surface layers of the water column, though inlate summer were highest at the thermocline. Thetiming of the spring bloom was variable. Aseasonal succession of phytoplankton species was observed. The winter phytoplankton assemblage was dominated bylarge benthic and chain diatoms and the summer assemblage byRhizosolenia species. Agreater number of dinoflagellates were present inthe stratified area. The zooplankton assemblage was dominated bythe copepod species Pseudocalanus elongatus, Acartia clausi, Temora longicomis and Oithona simtlts, peak abundances occurring inJuly. No spatial variation was evident inzooplankton densities and composition except for ichthyoplankton densities, which were greater inshore. No spatial differences were noted incopepod grazing rates, eggproduction rates or chemical composition. However, seasonal and interspecific differences were considerable, with ingestion rates highest during the spring bloom and grazing impact greatest during the summer. Copepods grazed only a small proportion of the chlorophyll a inthe water column. Maximum rates of egg production occurred in April, prior to the spring phytoplankton bloom, butwere also high in the summer. Interspecific differences inthe numbers of eggs produced bycopepods and intheir seasonal production patterns were large. Only a small portion of the populations were producing eggs at anyonetime. This individual variability was attributed to the age structure of the population. Copepods were consuming enough algae to cover the energetic costs of reproduction. A. clausi and T. longicornis had the highest rates of grazing and egg production. T. longtcomis actually had the greatest impact onthe ecosystem, despite being far less abundant than the other copepod species. High mortality inthejuvenile stages was hypothesised as the reason for its low adult abundances. Protein was the major chemical constituent of the zooplankton, then lipids with carbohydrates only forming a small percentage of the dry weight. The amount of each of these components peaked inthe summer, their relative percentage varying seasonally. The stratified site, being the deepest, had the highest standing stocks of plankton. It was calculated that copepod stocks and productivity were generally high enough to support the observed densities of ichthyoplankton in the region.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:307669 |
| Date | January 1995 |
| Creators | Nicholas, Kirsty Rebecca |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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