Climate-induced effects on the meroplankton and the benthic-pelagic ecology of the North Sea

Analyses of long-term time series of North Sea plankton and sea surface temperature (SST) data reveal that the annual planktonic larval abundance of three benthic phyla, Echinodermata, Arthropoda, and Mollusca, responds positively and immediately to SST. Long-term outcomes for the planktonic abundance of these three phyla are different, however. The planktonic larvae of echinoderms and decapod crustaceans have increased in abundance from 1958 to 2005, and especially since the mid-1980s, as North Sea SST has increased. In contrast, the abundance of bivalve mollusc larvae has declined, despite the positive year-to-year relationship between temperature and bivalve larval abundance continuing to hold. We argue that the changes in meroplankton abundance, coincident with increased phytoplankton and declining holoplankton, reflect the synchronous effect of rising SST and related changes in the pelagic community on the reproduction and recruitment of many benthic marine invertebrates. Under this scenario, the long-term decline in bivalve mollusc larvae will reflect increased predation on the settled larvae and adults by benthic decapods. These alterations in the zooplankton may therefore describe an ecosystem-wide restructuring of North Sea trophic interactions. Ocean warming over short timescales can bring about comprehensive changes in benthic communities (Schiel et al. 2004). There are few time series that contain data on a sufficient range of taxa to enable complex ecological climate-induced changes to be investigated at a community scale over the long term, however. The Continuous Plankton Recorder (CPR) survey (Batten et al. 2003) records up to 450 planktonic taxa routinely (Richardson et al. 2006) and maintains a long-term time series of North Atlantic and North Sea plankton community data that extends over 70 yr and is continuous in the North Sea since 1948. Samples of the North Sea plankton collected by the CPR survey on a monthly basis from 1958 to the present day reveal an increase in abundance of the meroplankton (the planktonic life history stages of benthic species) and a concordant decline in abundance of holoplankton taxa over this period (Lindley and Batten 2002). In particular, the larvae of benthic echinoderms, notably those of Echinocardium cordatum, now dominate numerically the North Sea plankton community in the summer (Kirby et al. 2007). The timing of peak occurrence of echinoderm larvae has also changed, advancing by 47 d over the same period (Edwards and Richardson 2004). Like other changes that have been reported in the distributions, abundances, and seasonal cycles of the North Sea and Atlantic plankton (Edwards et al. 2002), the changes in echinoderm larvae appear to be climate driven (Kirby et al. 2007). Although the larvae of benthic echinoderms have shown the greatest numerical increase and the largest seasonal advance among the plankton of the North Sea, changes in other meroplankton have also been observed (Lindley and Batten 2002). The other dominant meroplankton in CPR samples are the larvae of benthic decapods, bivalves, cirripedes, and ectoprocts (cyphonautes larvae) (5bryozoa). Although the greatest increase in North Sea sea surface temperature (SST) has occurred in the winter and spring months, SST is now higher throughout the year (Kirby et al. 2007). Fecundity in poikilotherms, egg size, and number of eggs are phenotypically plastic traits influenced by temperature; warmer temperatures during gametogenesis are usually associated with smaller eggs and larger clutch sizes (Fischer et al. 2003); in marine organisms this phenomenon is often represented by latitudinal clines or depth variation in reproductive output (Ramirez-Llodra 2002). In this way, increases in SST (sea bottom temperatures are close to SST in most continental shelf waters for much of the year) could influence reproductive output in a wide variety of organisms with similar life history strategies. 1 Corresponding author ([email protected]). Acknowledgments Richard R. Kirby is a Royal Society Research Fellow. We thank the owners, masters, and crews of the ships that tow the Continuous Plankton Recorder (CPR) on a voluntary basis. We are also grateful for the comments of the anonymous referees. Limnol. Oceanogr., 53(5), 2008, 1805–1815 E 2008, by the American Society of Limnology and Oceanography, Inc.