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.

Long-term changes in the meroplankton of the North Sea

Data from the continuous plankton recorder (CPR) survey collected in the late-1940s to early-1960s indicated that the abundance of decapod larvae was low and the seasonal peak of abundance was late following cold winters. The phenological effect of temperature was shown to be consistent with relationships between both geographical and interannual patterns of variation. Analyses of CPR data collected from the 1940s to the present day reveal large-scale long-term changes in the abundance and phenology of the North Sea meroplankton. Echinoderm larvae, whose peak abundance has advanced by 47 days, show the greatest shift in timing. Echinoderm larvae have also increased in abundance to become the most abundant taxon in North Sea CPR samples. Genetic and morphological analyses of CPR samples show that the variations in echinoderm larvae are mainly attributable to an increasing abundance and earlier occurrence of the larvae of a resident species, Echinocardium cordatum, rather than a change in species composition. The remarkable scale of the changes in abundance and phenology of the meroplankton, which are greater than those seen in the holoplankton, has stimulated the development of further research into the causes and effects of these changes.

Studies on meroplankton

Plankton community, drawn from a vary wide variety of animal phyla, formed the basic food supply of marine life and indicators of water mass. The term meroplankton generally referred to that portion of the zooplankton which is transient in nature, remaining rest of their lives in the nektonic or benthic environment. This group was selected for intensive studies, considering the role of meroplankton in the economy of the sea and the scarcity of literature on them from the Indian Ocean. The preser .udy besides providing information regarding the fixation and preservation !e _ iniques and biochemical aspects of tropical meroplankton, also consolidates information regarding their zoogeography in the Indian Ocean region, with a view to amplifying the limited information available from this area. The distribution studies are based on the collections made during the International Indian Ocean Expedition (1960-65), whereas the material for preservation and biochemical studies was collected from the coastal waters during 1968-1978. Salient features:- 2% of formaldehyde buffered with 2% borax, added to the plankton in the ratio of 9:1 was found the best fixative. On fixation the plankton underwent shrinkage due to loss of 15 to 87% water. Addition of antioxidants prevented colour fading. Narcotization by different specific reagents prior to fixation reduced distortions due to violent reaction and improved morphological conditions. One percent formaldehyde solution in sea water buffered with borax or neutralised with calcium carbonate perfectly preserved majority of meroplankton. Equally good was one percent propylene phenoxetol buffered with borax. Biochemical compostion of vaioous taxa showed variations according to their age class, size groups metamorphosing stage, feeding mechanism, type of organism fed and time of collection. General distribution studies of 4 meroplankton taxa - Anthozoan larvae, cirripedia larvae, sipunculoid larvae and gastropod larvae stowed abundance in the coastal areas especially during the SW monsoon period. Based on the larval distribution different zoo-geographical areas in the Indian Ocean are differentiated.

Meroplankton in the Kara Sea

Meroplankton was sampled at 11 stations in the southern Kara Sea and the Yenisei Estuary in September 2000. Larvae of 29 benthic taxa representing 10 higher groups were identified. Meroplankton was present at almost all stations and most depth levels. The two most abundant groups were Echinodermata (68%) and Polychaeta (26%). Echinoderms dominated total meroplankton locally due to mass occurrences of Ophiopluteus larvae. The relative group composition was highly variable and seemed to depend mainly on the local hydrographic pattern. Comparison of meroplanktonic data with the distribution of the adults revealed for Spionida and Bivalvia a 'downstream' transport of the larvae whereas for other polychaete species and Ophiuroida 'upstream' transport into the estuary occurred. The distribution and concentration of the larvae within the estuary is explained by physical barriers established by hydrographic gradients, the prevailing mixing processes and the presence of a near-bottom counter current.

Climate effects and benthic-pelagic coupling in the North Sea

The North Sea is one of the most biologically productive ecosystems in the world and supports important fisheries. Climate-induced changes occurred in the pelagic ecosystems of the North Sea during the 1980s. These changes, which have been observed from phytoplankton to fish and among permanent (holoplankton) and temporary (meroplankton) plankton species, have resulted in alterations in plankton community composition and seasonality. Until now, the effects of climate-driven changes on biological linkages between pelagic and benthic ecosystems have not been examined. The present study indicates that changes in benthic organisms could have a profound effect on the trophodynamics of the pelagos. We demonstrate this by analyses of a long-term time series of North Sea plankton and sea surface temperature data. We discover that pronounced changes in the North Sea meroplankton, mainly related to an increased abundance and spatial distribution of the larvae of a benthic echinoderm, Echinocardium cordatum, result primarily from a stepwise increase in sea temperature after 1987 that has caused warmer conditions to occur earlier in the year than previously. Key stages of reproduction in E. cordatum, gametogenesis and spawning, appear to be influenced by winter and spring sea temperature and their larval development is affected by the quantity and quality of their phytoplankton food. Our analyses suggest that a new thermal regime in the North Sea in winter and spring may have benefited reproduction and survival in this benthic species. As a result, E. cordatum may be altering the trophodynamics of the summer pelagic ecosystem through competition between its larvae and holozooplankton taxa.

Genetic analysis of Continuous Plankton Recorder (CPR) samples

Genetic analysis of Continuous Plankton Recorder (CPR) samples is enabling greater taxonomic resolution and the study of plankton population structure. Here, we present some results from the genetic analysis of CPR samples collected in the North Sea and north-eastern Atlantic that reveal the impacts of climate on benthic-pelagic coupling and the food web. We show that pronounced changes in the North Sea meroplankton are related to an increased abundance and spatial distribution of the larvae of the benthic echinoderm, Echinocardium cordatum. Key stages of reproduction in E. cordatum, gametogenesis and spawning, are influenced by winter and spring sea temperature (January-May). A stepwise increase in sea temperature after 1987, which has created warmer conditions earlier in the year, together with increased summer phytoplankton, may benefit the reproduction and survival of this benthic species. Competition between the larvae of E. cordatum and other holozooplanlcton taxa may now be altering the trophodynamics of the summer pelagic ecosystem. In the north-eastern Atlantic the genetic analysis of fish larvae sampled by the CPR has revealed an unprecedented increase in the abundance of juvenile snake pipefish, Entelurus aequoreiis since 2002. We argue that increased sea surface temperatures in winter and spring when the eggs of E. aqueoreus, which are brooded by the male, are developing and the young larvae are growing in the plankton are a likely cause. The increased abundance of this species in Atlantic and adjacent European seas already appears to be influencing the marine food web.

Long-term changes in North Atlantic zooplankton biomass

New measures of zooplankton biomass have been derived from CPR samples in the North Atlantic from 1958 to 2005. The final aim was to investigate how the zooplankton standing stock had varied throughout the last decades, knowing that in different areas of the North Atlantic significant changes in the distribution of the dominant zooplankton species as well as the plankton assemblage have been observed. During the forty-five years of monitoring the contribution of the different groups (e.g. copepods, euphausiids, meroplankton larvae) to the total zooplankton biomass has been evaluated. The changes in the phenology of the biomass were also considered. The relationship between quantity, quality and seasonal timing of plankton and the poor fish recruitment seen in recent years in the North Sea are also discussed.