Secondary production of coastal plankton communities in the western Irish Sea.
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
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
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.