Structure and flow of carbon and nitrogen to the western Irish Sea Nephrops norvegicus fishery: a stable isotope approach.

Abstract

Stable isotope analysis was used to describe the structure and flow of organic matter (carbon and nitrogen) from primary production in the water column to the benthos, and into secondary production that supports a fishery for the lobster Nephrops norvegicus in the western Irish Sea. There was strong seasonal variation in the carbon and nitrogen stable isotopes of organic matter associatedw ith the seasonalc ycle of primary production in the surfacew aters. The seasonal isotope signal was used to trace the flow of organic matter from primary production through herbivorous zooplankton to the carnivorous pelagic larvae of Nephrops norvegicus.T he production of larvae represented0 .2 % of spring bloom production. Stable isotope analysis of organic matter throughout the water column was used to track the vertical flux of primary production from the surface to the benthos. The isotope signal was also used to quantify the input of carbon to the benthos, estimated to be 41 % of spring bloom primary production. The carbon input to the benthos supported a simple food chain of three higher trophic levels (TL2 - TL4). Nephrops norvegicus was positioned at TU where it accounted for 96 % of the total biomass at that level. The input of carbon to the benthos was equal to the removal of carbon from the benthos by the Nephrops norvegicus fishery assuming a transfer efficiency of 16 % between each trophic level. Climate warming has caused a decline in primary production in some marine systems. Should similar impacts occur in the western Irish Sea,t he subsequendt ecline in the flux of carbon to the benthos will result in a shortfall in the supply of carbon needed to support the current catch rate of the Nephrops norvegicus fishery.