Population dynamics and secondary production of the small copepods in the Menai Strait
Measuring zooplankton standing stock and production is a central problem in marine biology since zooplankton, particularly copepods, link primary productivity to fisheries productivity. The aim of the present study was to determine the seasonal and annual variation in the standing stock and secondary production of the main calanoid copepod species found in the Menai Strait, eastern Irish Sea. The zooplankton survey was carried out between January 1996 and December 1997. In addition, the reproduction and the respiration rate of the dominant copepod species, T. longicornis, was investigated between 1996 and 1998 during field and laboratory experiments to study its population dynamics. The temperature during January through to April differed between years, with the winter 1996 being colder than that of 1997 and 1998. The timing of the spring phytoplankton bloom also differed between years with the Chla maximum in 1996 occurring -1 month later than in 1997 and 1998. T. longicornis produced eggs all year round with maximum carbon-specific egg production rates (EPR) of (-0.14 μg Egg-C jig fem. -C-1 day 1) coinciding with the spring phytoplankton bloom and minimum rates (-0.01 μg Egg-C μg fem. -C-1 day1) in winter. The pattern of natural EPR variability indicates that individual fecundity was positively related to female weight and food quantity (possibly constrained by food size or quality) and negatively related to tidal range (i. e. total suspended sediment). In all three years, the hatching success (% HS) of the eggs laid decreased by -80 % during peak phytoplankton production and was significantly negatively correlated (r = -0.47, p<0.05, d. f. = 96) with ambient Chl-a concentration. These non-hatching eggs could have been diapause eggs. The respiration rate of T. longicornis varied during the year increasing both with body weight and with temperature. The metabolic daily energy loss of an adult copepod account for between 4% and 8% of its body carbon (winter and summer respectively). The seasonal pattern of copepod abundance and species composition was typical of temperate coastal areas. The seasonal variation in the copepod community (by number and biomass) showed T. longicornis to be the most abundant in spring, Centropages hamatus and Acartia clausi in summer and Pseudocalanus sp. in autumn and winter. Maximum total copepod standing stock occurred during the spring phytoplankton bloom and minimum between autumn and winter with total annual standing stock in 1996 (618 mg-C m 3) being -4 times lower than in 1997 (2530 mg-C m-3). Stage specific, copepod cephalothorax lengths varied with season and in most cases were negatively correlated with temperature. Individual weights and abundance of the copepods, together with measures of temperature, were used to predict weight specific growth and production rates using published empirical relationships. Calanoid copepod total annual production varied between 37-160 mg-C M-3 yr 1 for 1996 and 1997 respectively with T. longicornis accounting for -50 % of the total followed by C. hamatus (-25 %), A. clausi (-20 %) and Pseudocalanus sp. (5 %). Annual carbon flow in the Menai Strait was estimated from copepod production with measures of primary production, production of bacteria (previous study) and ciliates at this site. It is suggested that since the spring increase in T. longicornis population could not be attributed to the EPR of over-wintering females alone, the excess of copepods may either originate from the hatching of resting eggs during winter or from transport of animals from southern regions. If resting eggs were implicated in copepod population dynamics the annual variation in copepod standing stock may be controlled by climate change through differential hatching rate of resting eggs in winter.