Transregional linkages in the North-Eastern Atlantic - An ‘end-to-end ’ analysis of pelagic ecosystems

This review examines interregional linkages and gives an overview perspective on marine ecosystem functioning in the north-eastern Atlantic. It is based on three of the 'systems' considered by the European Network of Excellence for Ocean Ecosystems Analysis (EUR-OC EANS was established in 2004 under the European Framework VI funding programme to promote integration of marine ecological research within Europe), the Arctic and Nordic Seas, North Atlantic shelf seas and North Atlantic. The three systems share common open boundaries and the transport of water, heat, nutrients and particulates across these boundaries modifies local processes. Consistent with the EUR-OC EANS concept of 'end-to-end' analyses of marine food webs, the review takes an integrated approach linking ocean physics, lower trophic levels and working up the food web to top predators such as marine mammals. We begin with an overview of the regions focusing on the major physical patterns and their implications for the microbial community, phytoplankton, zooplankton, fish and top predators. Human-induced links between the regional systems are then considered and finally possible changes in the regional linkages over the next century are discussed. Because of the scale of potential impacts of climate change, this issue is considered in a separate section. The review demonstrates that the functioning of the ecosystems in each of the regions cannot be considered in isolation and the role of the atmosphere and ocean currents in linking the North Atlantic Ocean, North Atlantic shelf seas and the Arctic and Nordic Seas must be taken into account. Studying the North Atlantic and associated shelf seas as an integrated 'basin-scale' system will be a key challenge for the early twenty-first century. This requires a multinational approach that should lead to improved ecosystem-based approaches to conservation of natural resources, the maintenance of biodiversity, and a better understanding of the key role of the north-eastern Atlantic in the global carbon cycle.

Spawning season and temperature relationships for sardine (Sardina pilchardus) in the eastern North Atlantic

Spawning temperature preferences for sardine (Sardina pilchardus) in the eastern North Atlantic were determined from field data. These were compared with climatological temperature cycles (1986-2002) derived from satellite data by geographical region, to predict spawning seasons. Optimum spawning temperatures were determined as 14.0-15.0oC from the English Channel to Portugal and 16.0–18.0oC for all north-west African regions. Spawning seasons were closely related to the general latitudinal trend of the annual temperature cycle, with modification by upwelling in the western Iberian and north-west African regions. Some differences between temperature-based spawning season predictions and field observations were related to variations in seasonal plankton production. Correlations in the annual time-series of favourable spawning temperatures suggested relatively strong linkages between the southern areas from Portugal to Senegal. There was no consistent relationship between annual variations in duration of temperature-predicted spawning seasons and observed field abundance of eggs.

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.

Regime shifts in marine ecosystems: detection prediction and management

Regime shifts are abrupt changes between contrasting, persistent states of any complex system. The potential for their prediction in the ocean and possible management depends upon the characteristics of the regime shifts: their drivers (from anthropogenic to natural), scale (from the local to the basin) and potential for management action (from adaptation to mitigation). We present a conceptual framework that will enhance our ability to detect, predict and manage regime shifts in the ocean, illustrating our approach with three well-documented examples: the North Pacific, the North Sea and Caribbean coral reefs. We conclude that the ability to adapt to, or manage, regime shifts depends upon their uniqueness, our understanding of their causes and linkages among ecosystem components and our observational capabilities.

The response of phytoplankton to climate variability associated with the North Atlantic Oscillation

Inter-annual variability in the timing of phytoplankton spring bloom and phytoplankton community structure in the central North Atlantic Ocean was quantified using ocean color data and continuous plankton recorder (CPR) data. This variability was related to the North Atlantic Oscillation using correlation analysis and multivariate auto-regression models. The initiation of the spring bloom derived from CPR phytoplankton color index data is similar to that derived from satellite chlorophyll, and exhibits a nominal correlation with the sea surface temperature (SST) and the North Atlantic Oscillation (NAO). The extrapolated spring bloom timing suggested later initiation of blooms in the mid-1980s and earlier initiation of blooms in the 1990s. The climatological phytoplankton community structure in the central North Atlantic is dominated by diatoms, except for a shift in community composition favoring dinoflagellates in August. The ratio of diatoms to total phytoplankton abundance and the ratio of dinoflagellates to total phytoplankton abundance are both closely correlated with the NAO and SST. The extended time series of phytoplankton community structure between 1985 and 2009, deduced from the time series of SST and NAO over the same interval, showed a decadal shift away from diatoms towards dinoflagellates. The linkages between the NAO, and changes in stratification and phytoplankton processes occur over a larger scale than previously observed.