[show abstract][hide abstract] ABSTRACT: The North Sea cod (Gadus morhua, L.) stock has continuously declined over the past four decades linked with overfishing and climate change. Changes in stock structure due to overfishing have made the stock largely dependent on its recruitment success, which greatly relies on environmental conditions. Here we focus on the spatio-temporal variability of cod recruitment in an effort to detect changes during the critical early life stages. Using International Bottom Trawl Survey (IBTS) data from 1974 to 2011, a major spatio-temporal change in the distribution of cod recruits was identified in the late 1990s, characterized by a pronounced decrease in the central and southeastern North Sea stock. Other minor spatial changes were also recorded in the mid-1980s and early 1990s. We tested whether the observed changes in recruits distribution could be related with direct (i.e. temperature) and/or indirect (i.e. changes in the quantity and quality of zooplankton prey) effects of climate variability. The analyses were based on spatially-resolved time series, i.e. sea surface temperature (SST) from the Hadley Center and zooplankton records from the Continuous Plankton Recorder Survey. We showed that spring SST increase was the main driver for the most recent decrease in cod recruitment. The late 1990s were also characterized by relatively low total zooplankton biomass, particularly of energy-rich zooplankton such as the copepod Calanus finmarchicus, which have further contributed to the decline of North Sea cod recruitment. Long-term spatially-resolved observations were used to produce regional distribution models that could further be used to predict the abundance of North Sea cod recruits based on temperature and zooplankton food availability.
PLoS ONE 01/2014; 9(2):e88447. · 3.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Increasing pressure on animal populations through climate change and anthropogenic exploitation fuel the need for understanding complex life cycle dynamics of key ecosystem species and their responses to external factors. Here we provide a novel, integrative study on long-term population dynamics of Pseudocalanus acuspes, a key species in the Baltic Sea, explicitly considering its distinct life history stages and testing for linear, non-linear and non-additive climate and food web effects. Based on a unique data set of stage specific abundance covering almost five decades of sampling (1960 – 2008, 1408 samples), we use Generalized Additive Modeling (GAM) and its respective non-additive threshold (TGAM) formulation to test for (i) density effects on subsequent life history stages within the internal life cycle, and (ii) the effect of exogenous bottom-up (i.e. hydro-climatic) and top-down (i.e. predation) pressures, and (iii) changes between bottom-up to top-down regulation. We show that linear density effects are always present explaining a high proportion of interannual variability, while effects of external pressures are non-linear or non-additive and strongly stage and season specific. In general, younger stages of P. acuspes are more affected by atmospheric winter conditions and water temperature whereas older stages are influenced by conditions of deepwater salinity and predation pressure. These bottom-up processes, however, are not necessarily stable and can depend on the level of top-down predation pressure. Our study demonstrates the complex and non-stationary interplay between internal and external factors regulating long-term animal population dynamics.
Marine Ecology Progress Series 11/2013; · 2.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: Understanding the effects of cross-system fluxes is fundamental in ecosystem ecology and biological conservation. Source-sink dynamics and spillover processes may link adjacent ecosystems by movement of organisms across system boundaries. However, effects of temporal variability in these cross-system fluxes on a whole marine ecosystem structure have not yet been presented. Here we show, using 35 y of multitrophic data series from the Baltic Sea, that transitory spillover of the top-predator cod from its main distribution area produces cascading effects in the whole food web of an adjacent and semi-isolated ecosystem. At varying population size, cod expand/contract their distribution range and invade/retreat from the neighboring Gulf of Riga, thereby affecting the local prey population of herring and, indirectly, zooplankton and phytoplankton via top-down control. The Gulf of Riga can be considered for cod a "true sink" habitat, where in the absence of immigration from the source areas of the central Baltic Sea the cod population goes extinct due to the absence of suitable spawning grounds. Our results add a metaecosystem perspective to the ongoing intense scientific debate on the key role of top predators in structuring natural systems. The integration of regional and local processes is central to predict species and ecosystem responses to future climate changes and ongoing anthropogenic disturbances.
Proceedings of the National Academy of Sciences 04/2012; 109(21):8185-9. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this chapter, the temporal variability in nutrient concentrations (nitrate, silicate and phosphate)at different depths along the northwestern (Galicia) and northern (mar Cantábrico) Spanish mid-shelf (75-130 m isobath) was analyzed. To this aim, we have considered the time-series of monthly observations in Galicia (Vigo and A Coruña) and in the mar Cantábrico (Cudillero, Gijón and Santander), sampled since the decade of 1990 to 2007, in the frame of the project RADIALES. Nutrient concentrations were analyzed taking into account physical (temperature, salinity and density), biological (chlorophyll-a) and meteo-climatic variables that modulate this variability trough processes such as coastal upwelling, the Iberian Poleward Current (IPC), continental runoff and phytoplankton production. The long-term climatic averages decrease eastward along the continental shelf between the south of Galicia and the eastern mar Cantábrico. In general, seasonality is the main mode of temporal variability of the time series. The percentage of variance associated to this component (i .e. an indication of its predictability) is higher in the mar Cantábrico than in Galicia, while its amplitude is higher and the timing of the maxima seasonal concentrations occur earlier during the year in Galicia than in the mar Cantábrico. Despite the significant increasing trends in some of the nitrate and phosphate series, both in Galicia and in the mar Cantábrico, these long-term trends are highly influenced by the presence of outliers (i. e. large positive anomalies), which stress the importance of the monitoring long time-series to assess the robustness of the estimated long-term changes. As deduced from the stoichiometric nutrient ratios, nitrate, followed by phosphate, appears as the limiting nutrient for primary production. In turn, high silicate concentrations in Galicia agree with the higher predominance of diatoms in this region relative to the mar Cantábrico. For each location, the high correlations for a given variable between water layers indicate a good transmission of nutrient anomalies (i.e. enrichment or uptake) through the water column. Contrastingly, the low along-shelf spatial correlation stress the importance of local versus mesoscale or regional factors in determining the variability of nutrient concentrations at each location. Wind stress, mainly through upwelling, greatly affects nutrient fertilization along the shelf. Its effect, however, is larger in Galicia than in the mar Cantábrico. Similarly, the climatic conditions which promote upwelling are positively correlated to nutrients in Galicia while those implying an increasing subtropical influence display negative correlations. In contrast, the transitional character of the mar Cantábrico would explain the lack of significant correlations between nutrients and climatic indices in this area.
[show abstract][hide abstract] ABSTRACT: It is well known that human activities, such as harvesting, have had major direct effects on marine ecosystems. However, it is far less acknowledged that human activities in the surroundings might have important effects on marine systems. There is growing evidence suggesting that major reorganization (i.e., a regime shift) is a common feature in the temporal evolution of a marine system. Here we show, and quantify, the interaction of human activities (nutrient upload) with a favourable climate (run-off) and its contribution to the eutrophication of the Black Sea in the 1980s. Based on virtual analysis of the bottom-up (eutrophication) vs. top-down (trophic cascades) effects, we found that an earlier onset of eutrophication could have counteracted the restructuring of the trophic regulation at the base of the food web that resulted from the depletion of top predators in the 1970s. These enhanced bottom-up effects would, however, not propagate upwards in the food web beyond the zooplankton level. Our simulations identified the removal of apex predators as a key element in terms of loss of resilience that inevitably leads to a reorganization. Once the food web has been truncated, the type and magnitude of interventions on the group replacing the apex predator as the new upper trophic level have no effect in preventing the trophic cascade. By characterizing the tipping point at which increased bottom-up forcing exactly counteracts the top-down cascading effects, our results emphasize the importance of a comprehensive analysis that take into account all structuring forces at play (including those beyond the marine system) at a given time.
[show abstract][hide abstract] ABSTRACT: In order to provide better fisheries management and conservation decisions, there is a need to discern the underlying relationship between the spawning stock and recruitment of marine fishes, a relationship which is influenced by the environmental conditions. Here, we demonstrate how the environmental conditions (temperature and the food availability for fish larvae) influence the stock-recruitment relationship and indeed what kind of stock-recruitment relationship we might see under different environmental conditions. Using unique zooplankton data from the Continuous Plankton Recorder, we find that food availability (i.e. zooplankton) in essence determines which model applies for the once large North Sea cod (Gadus morhua) stock. Further, we show that recruitment is strengthened during cold years and weakened during warm years. Our combined model explained 45 per cent of the total variance in cod recruitment, while the traditional Ricker and Beverton-Holt models only explained about 10 per cent. Specifically, our approach predicts that a full recovery of the North Sea cod stock might not be expected until the environment becomes more favourable.
Proceedings of the Royal Society B: Biological Sciences 02/2011; 278(1705):504-10. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Increased knowledge on the spatial distribution of marine resources is crucial for the implementation of a true ecosystem approach to management and the conservation of marine organisms. For exploited fish species characterized by aggregation behaviour during spawning time, the identification and tracking of spawning areas is essential for a correct assessment of their productivity and population abundance. To elucidate this concept, we reconstructed the spatio-temporal distribution of adult plaice (Pleuronectes platessa, Pleuronectidae) during spawning time along the 20th century. Historical data reveal that not only the abundance but also the former population richness was much higher than previously estimated and has declined because of protracted over-exploitation during the last 30 years. We conclude that forecast of stock recovery to former levels of abundance neglecting spatial reorganizations might be over-optimistic and shaded by a lost memory of the past population richness. These results reinforce the importance of managing exploited marine resources at a greater spatial resolution than has been carried out in the history of fishery management.
Fish and Fisheries 11/2010; 12(3):289 - 298. · 5.86 Impact Factor
[show abstract][hide abstract] ABSTRACT: The nature and periodicity of changes in the concentrations of inorganic nutrients, phytoplankton chlorophyll-biomass and primary production rates at annual time scales were investigated along the NW shelf of Spain. The time series analyzed included monthly in-situ measurements obtained at mid-shelf (80-100 m depth) stations at 4 locations along the coast between 1990 and 2007. In addition satellite-derived chlorophyll data covering the period 1998-2007 were also studied. Local and large scale forcing mechanisms were included in the form of upwelling offshore water transport and climatic indices, respectively. Linear trends, changes in seasonality and multiyear cycles were different in each location. All sites showed increasing sea surface temperature and decreasing upwelling intensity, along with a sensible increase in surface stratification, more pronounced in the northern (S Bay of Biscay) than in the western sector (Galicia). As a result, there was a marked decrease in nutrient concentrations, chlorophyll and primary production in the former while the trends were barely significant in the later. Multiannual cycles in nutrient concentrations with periodicity between 3.5 and 9 years were found. Such non-linear changes affect the interpretation of long-term trends in nutrient supply and primary production as primary effects of climate-driven changes in this ecosystem.
[show abstract][hide abstract] ABSTRACT: The southern Bay of Biscay is a very active region in terms of hydrography due to the interannual variations of its Central Waters, the recurrence of mesoscale features such as slope currents and upwellings, and the freshwater discharges from land. This highly dynamic physical environment influences to a great extent the biogeochemical cycles of nutrients beyond the seasonal cycle typical of middle latitudes. By using a monthly time series (1993-2003) of nitrate, nitrite, phosphate, and silicate consisting of three stations placed along a cross-shelf transect, we assess the role of the physical forcing on nutrient seasonal and interannual dynamics within the upper 200 m, as well as the interactions with the biological component. The seasonal cycles of all nutrients and the stoichiometric balances (N:P and Si:N) are characterized along this coastal-oceanic gradient. The year-to-year variations in the extent of the winter replenishment are analyzed in relation to the background Central Waters and presence/absence of the Iberian Poleward Current. In the long term we report decreasing linear trends of nitrate, nitrite, and silicate as well as an uncoupled nonlinear variation (i.e., cyclical) for all nutrients. Furthermore, we investigate the effect of this complex long-term forcing on the phytoplankton: the linear trends are probably related to a decreasing primary production rate, while the nonlinear forcing may be responsible for controlling the community structure of phytoplankton.
Journal of Geophysical Research 01/2007; 112. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: The project “Studies on time series of oceanographic data” was established as a pilot project by the Instituto Español de Oceanografía (I.E.O.) in 1991. After more than a decade, the project has grown to encompass a network of 19 sampling stations in five different transects along the North and Northwest coast of Spain: Vigo, Coruña, Cudillero, Gijón and Santander beginning in 1987, 1988, 1993, 2001, 1991, respectively. At each location a coastal-ocean gradient is sampled monthly for hydrography, nutrients and planktonic communities. We have used these data to set with statistical significance the range of variability of several environmental variables and biological communities and determined the rates and trends of warming due to climate change as well as to describe some direct and indirect effects of the increase in water temperature on the pelagic ecology. The project had substantially contributed to get a deeper knowledge on planktonic communities and species and to produce baselines, climatologies and reference levels for the North coast of Spain, which allow us to do accurate evaluations on the effects of environmental perturbations on the ecosystem and forecast the expected recovery time. These changes both in the physical structure of the water column and in the trophic level that drives ecosystem production and functioning are likely to also modify the structure, production and organization of higher trophic levels like zooplankton. Our analysis shows that only through sustained and repeated time series sampling it is feasible to detect these changes. Indeed, the annual cycle of zooplankton biomass seems to be restricted in time, with the annual decrease in zooplankton biomass matching the onset of stratification. The observed patterns in the seasonal occurrence of incoming species like Temora stylifera are related to those observed in the water column stratification, which is reinforced by the warming trend.
Progress In Oceanography 01/2007; · 3.71 Impact Factor
[show abstract][hide abstract] ABSTRACT: Much interest has recently been devoted to reconstructing the dynamic structure of ecological systems on the basis of time-series data. Using 10 years of monthly data on phyto- and zooplankton abundance from the Bay of Biscay (coastal to shelf-break sites), we demonstrate that the interaction between these two plankton components is approximately linear, whereas the effects of environmental factors (nutrients, temperature, upwelling and photoperiod) on these two plankton population growth rates are nonlinear. With the inclusion of the environmental factors, the main observed seasonal and inter-annual dynamic patterns within the studied plankton assemblage also indicate the prevalence of bottom-up regulatory control.
Proceedings of the Royal Society B: Biological Sciences 12/2006; 273(1603):2831-8. · 5.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The southern Bay of Biscay (NW Spain) shows a very active hydrography due to the different origins of its Central Waters, the local modifications exerted on them by continental effects and the recurrence of mesoscale processes such as slope currents, upwellings and eddies. In order to assess the role of the different sources of variability we conducted a monthly series of CTD sampling in the central Cantabrian Sea along a coastal-oceanic transect, from 1993 to 2003. We analyzed the spatial variability of the hydrographic processes over different timescales. The thermohaline properties of Central Waters varied between those typical of the subpolar mode of the Eastern North Atlantic Central Water (ENACWsp) and a local mode, the Bay of Biscay Central Water (BBCW), though there has been a clear shift toward the BBCW prevalence in the last years. The Iberian Poleward Current (IPC) conveyed subtropical Central Waters (ENACWst) into the region almost every winter. This slope current may display a double-core structure during some extreme events. The upper layers of the ocean showed a long-term trend toward increasing temperature and decreasing salinity, and accordingly density was on the decrease. These patterns suggest an enhancement of the water column stratification. Coastal upwellings are an important source of inshore variability and counteract these long-term changes on the coast. However, their intensity seems to be decreasing and their seasonal pattern changing toward a general advancement of the upwelling-favorable season.
Journal of Geophysical Research 01/2006; 111. · 3.17 Impact Factor
[show abstract][hide abstract] ABSTRACT: Changes phytoplankton variables were studied during spring 2003, after the Prestige shipwreck, in the N-NW Spanish coast using historical data available for the area. Some minor changes were occasionally observed, but they did not show any clear pattern and were related to natural variability of the ecosystem. The phytoplankton community structure did not change. The higher abundances of a few species were related to the high temperatures measured during 2003 that resulted in an advance of summer-like conditions to spring. The lack of effects is discussed in relation to the mechanisms removing fuel from surface waters to the sea floor. Zooplankton and diatoms play a key role in removing both oil particles and emulsions form surface waters. Presently, it is not possible to determine even some minor effects on plankton because the large variability of phytoplanktonic cycles.