Spatial and temporal abundance patterns for the late stage copepodites of Metridia lucens (Copepoda: Calanoida) in the US northeast continental shelf ecosystem

The annual cycle of abundance for the calanoid copepod Metridialucens and its interannual variability are described from 17611 samples collected on 193 broad-scale plankton surveys ofthe US northeast continental shelf region from 1977 to 1999.The copepod’s seasonal distribution is illustrated withbi-monthly geographic abundance plots. Abundance of M. lucensis compared with various environmental variables, abundanceof the copepod Calanus finmarchicus, density of potential predatorsand the North Atlantic Oscillation (NAO) index. Metridia lucensabundance increased throughout the ecosystem during spring warmingand usually reached peak levels in late spring. The highestmean densities were measured in the Gulf of Maine (GOM), wherethe copepod is usually found year round and exhibits a weak,decreasing west to east abundance gradient. The less dense populationslocated on Georges Bank and within the Middle Atlantic Bightare characterized by year-round onshore to offshore gradientsthat strengthen during the summer months when the copepod concentratesoffshore where there is a cool layer of subthermocline water,the region often referred to as the cold pool. Interannual variabilityin abundance showed two persistent periods: above average valuesfrom 1979 to 1982, followed by very low levels from 1983 toat least 1987. Metridia lucens abundance in the 1990s has usuallybeen at or above time series means. None of the environmentalvariables measured during the surveys could be correlated tothis variability. The copepod’s abundance in the GOM wasnot related to variation in the NAO index or to changes in predatorabundance found in the region. The seasonal abundance cycleand interannual variability of M. lucens in the GOM were foundto be very similar to those found for the dominant copepod,Calanus finmarchicus. Reasons for this coupling are discussed.