[Show abstract][Hide abstract] ABSTRACT: Background / Purpose:
Main environmental drivers of the seasonal life stage abundances of Pseudocalanus acuspes in combination with internal density effects were assessed to improve the understanding of its life cycle dynamics. For this, Generalized Additive Models (GAMs) were applied within a discrete stage-structured population modelling approach. In addition, GAMs were extended by including threshold formulations (TGAMs) to test for changes between bottom-up and top-down trophic regulation under contrasting predation regimes.
The integrative analysis revealed a complex interplay of linear density and predation effects and non-linear hydro-climate effects, the latter being stage- and season-specific. In general, younger stages were more affected by temperature and the local climate, while older stages were influenced by salinity, except in winter. Predation had a significant, negative effect on older stages throughout the year and controlled the bottom-up effects (i.e. density and salinity) mainly in summer and autumn on the older copepodites.
98th Ecological Society of America Annual Meeting 2013; 03/2014
[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. · 3.53 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; · 2.64 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. · 9.81 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.
01/2012: chapter 4: pages 121 – 158; Instituto Español de Oceanografía., ISBN: 9788495877086
[Show abstract][Hide abstract] ABSTRACT: Traditionally, marine ecosystem structure was thought to be bottom‐up controlled. In recent years, a number of studies have highlighted the importance of top‐down regulation. Evidence is accumulating that the type of trophic forcing varies temporally and spatially, and an integrated view – considering the interplay of both types of control – is emerging. Correlations between time series spanning several decades of the abundances of adjacent trophic levels are conventionally used to assess the type of control: bottom‐up if positive or top‐down if this is negative. This approach implies averaging periods which might show time‐varying dynamics and therefore can hide part of this temporal variability. Using spatially referenced plankton information extracted from the Continuous Plankton Recorder, this study addresses the potential dynamic character of the trophic structure at the planktonic level in the North Sea by assessing its variation over both temporal and spatial scales. Our results show that until the early‐1970s a bottom‐up control characterized the base of the food web across the whole North Sea, with diatoms having a positive and homogeneous effect on zooplankton filter‐feeders. Afterwards, different regional trophic dynamics were observed, in particular a negative relationship between total phytoplankton and zooplankton was detected off the west coast of Norway and the Skagerrak as opposed to a positive one in the southern reaches. Our results suggest that after the early 1970s diatoms remained the main food source for zooplankton filter‐feeders east of Orkney–Shetland and off Scotland, while in the east, from the Norwegian Trench to the German Bight, filter‐feeders were mainly sustained by dinoflagellates.
Global Change Biology 01/2012; 18(1). · 8.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Background/Question/Methods
European anchovy (Engraulis encrasicolus) populations of the North Sea increased dramatically since the 1990s, while usually being present only further South like the Bay of Biscay or Mediterranean Sea. This poses the questions of what ecosystem effects this will engender and why this change occurred when it did. Temperature and food availability changes are likely candidates to address the latter, and for the former, impacts on habitat and food availability for other planktivores such as the highly commercial herring (Clupea harengus) are the most pressing. Herring has shown low recruitment since 2000 and the role of changed abundance of other prey or potential competitor species is not well understood. We combine empirical work and spatio-temporal statistical modeling to address these questions. The composition and diversity in diets of anchovy, and two co-occurring clupeids: sprat (Sprattus sprattus) and herring were compared based on stomach analyses. Furthermore, the distribution in space and time of anchovy is compared to that of its likely prey items using generalized additive modeling on data from the Continuous Plankton Recorder (curtesy of SAHFOS) and the International Bottom Trawl Survey (curtesy of ICES).
Anchovy diet was more diverse at both the population and at the individual level than the diets of sprat and herring respectively, indicating that anchovy may be able to feed on a more variable plankton environment than the other species. The distribution of anchovy in the North Sea was best explained by temperature and spatial effect, while biomass of zooplankton, and abundance of major prey items did not explain anchovy distribution. We thus conclude that the reason for anchovy increase in the North Sea is not related to food availability, and its impact on potential competitors is unlikely to be related to food. Habitat interactions may play a role that needs to be further elucidated using the sparse data available on this new and uncommercialized species in the North Sea.
[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 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: 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 Atmospheres 01/2007; 112. · 3.44 Impact Factor