32 research outputs found
Tidal impacts on primary production in the North Sea
This study highlights the importance of tides in controlling the spatial and
temporal distributions of phytoplankton and other factors related to growth,
such as nutrients and light availability. To quantify the responses of net
primary production (NPP) to tidal forcing, we conducted scenario model
simulations considering M2 and S2 tidal constituents using the
physical–biogeochemical coupled model ECOSMO (ECOSystem MOdel). The results were
analyzed with respect to a reference simulation without tidal forcing, with
particular focus on the spatial scale of the tidally induced changes. Tidal
forcing regulates the mixing–stratification processes in shelf seas such as
the North Sea and hence also influences ecosystem dynamics. In principle, the
results suggest three different response types with respect to primary
production: (i) in southern shallow areas with strong tidal energy
dissipation, tidal mixing dilutes phytoplankton concentrations in the upper
water layers and thereby decreases NPP. Additionally, tides increase
turbidity in near-coastal shallow areas, which has the potential to further
hamper NPP. (ii) In the frontal region of the southern North Sea, which is a
transition zone between stratified and mixed areas, tidal mixing infuses
nutrients into the surface mixed layer and resolves summer nutrient
depletion, thus sustaining the NPP during the summer season after spring
bloom nutrient depletion. (iii) In the northern North Sea, the NPP response
to tidal forcing is limited. Additionally, our simulations indicate that
spring bloom phenology is impacted by tidal forcing, leading to a later onset
of the spring bloom in large parts of the North Sea and to generally higher
spring bloom peak phytoplankton biomasses. By testing the related changes in
stratification, light conditions and grazing pressure, we found that all
three factors potentially contribute to the change in spring bloom phenology
with clear local differences. Finally, we also analyzed the impact of the
spring–neap tidal cycle on NPP. The annual mean impact of spring–neap tidal
forcing on NPP is limited. However, locally, we found substantial differences
in NPP either in phase or anti-phase with the spring–neap tidal cycle. These
differences could be attributed to locally different dominant factors such as
light or nutrient availability during spring tides. In general, we conclude
that in shallow shelf seas such as the North Sea, intensified vertical mixing
induced by tidal forcing could either promote NPP by counteracting nutrient
depletion or hinder NPP by deteriorating the light environment because of the
resuspension and mixing of suspended matter into the euphotic zone.</p
Compound flood events: analysing the joint occurrence of extreme river discharge events and storm surges in northern and central Europe
The simultaneous occurrence of extreme events gained more and more attention from scientific research in the last couple of years.
Compared to the occurrence of single extreme events, co-occurring or compound extremes may substantially increase risks.
To adequately address such risks, improving our understanding of compound flood events in Europe is necessary and requires reliable estimates of their probability of occurrence together with potential future changes.
In this study compound flood events in northern and central Europe were studied using a Monte Carlo-based approach that avoids the use of copulas.
Second, we investigate if the number of observed compound extreme events is within the expected range of 2 standard deviations of randomly occurring compound events. This includes variations of several parameters to test the stability of the identified patterns. Finally, we analyse if the observed compound extreme events had a common large-scale meteorological driver.
The results of our investigation show that rivers along the west-facing coasts of Europe experienced a
higher amount of compound flood events than expected by pure chance.
In these regions, the vast majority of the observed compound flood events seem to be related to the cyclonic westerly general weather pattern (Großwetterlage).</p
Potential impacts of climate change on the primary production of regional seas: A comparative analysis of five European seas
Regional seas are potentially highly vulnerable to climate change, yet are the most directly societally important regions of the marine environment. The combination of widely varying conditions of mixing, forcing, geography (coastline and bathymetry) and exposure to the open-ocean makes these seas subject to a wide range of physical processes that mediates how large scale climate change impacts on these seas’ ecosystems. In this paper we explore the response of five regional sea areas to potential future climate change, acting via atmospheric, oceanic and terrestrial vectors. These include the Barents Sea, Black Sea, Baltic Sea, North Sea, Celtic Seas, and are contrasted with a region of the Northeast Atlantic. Our aim is to elucidate the controlling dynamical processes and how these vary between and within these seas. We focus on primary production and consider the potential climatic impacts on: long term changes in elemental budgets, seasonal and mesoscale processes that control phytoplankton’s exposure to light and nutrients, and briefly direct temperature response. We draw examples from the MEECE FP7 project and five regional model systems each using a common global Earth System Model as forcing. We consider a common analysis approach, and additional sensitivity experiments.
Comparing projections for the end of the 21st century with mean present day conditions, these simulations generally show an increase in seasonal and permanent stratification (where present). However, the first order (low- and mid-latitude) effect in the open ocean projections of increased permanent stratification leading to reduced nutrient levels, and so to reduced primary production, is largely absent, except in the NE Atlantic. Even in the two highly stratified, deep water seas we consider (Black and Baltic Seas) the increase in stratification is not seen as a first order control on primary production. Instead, results show a highly heterogeneous picture of positive and negative change arising from complex combinations of multiple physical drivers, including changes in mixing, circulation and temperature, which act both locally and non-locally through advection
Effects of Nutrient Management Scenarios on Marine Eutrophication Indicators: A Pan-European, Multi-Model Assessment in Support of the Marine Strategy Framework Directive
A novel pan-European marine model ensemble was established, covering nearly all seas under the regulation of the Marine Strategy Framework Directive (MSFD), with the aim of providing a consistent assessment of the potential impacts of riverine nutrient reduction scenarios on marine eutrophication indicators. For each sea region, up to five coupled biogeochemical models from institutes all over Europe were brought together for the first time. All model systems followed a harmonised scenario approach and ran two simulations, which varied only in the riverine nutrient inputs. The load reductions were evaluated with the catchment model GREEN and represented the impacts due to improved management of agriculture and wastewater treatment in all European river systems. The model ensemble, comprising 15 members, was used to assess changes to the core eutrophication indicators as defined within MSFD Descriptor 5. In nearly all marine regions, riverine load reductions led to reduced nutrient concentrations in the marine environment. However, regionally the nutrient input reductions led to an increase in the non-limiting nutrient in the water, especially in the case of phosphate concentrations in the Black Sea. Further core eutrophication indicators, such as chlorophyll-a, bottom oxygen and the Trophic Index TRIX, improved nearly everywhere, but the changes were less pronounced than for the inorganic nutrients. The model ensemble displayed strong consistency and robustness, as most if not all models indicated improvements in the same areas. There were substantial differences between the individual seas in the speed of response to the reduced nutrient loads. In the North Sea ensemble, a stable plateau was reached after only three years, while the simulation period of eight years was too short to obtain steady model results in the Baltic Sea. The ensemble exercise confirmed the importance of improved management of agriculture and wastewater treatments in the river catchments to reduce marine eutrophication. Several shortcomings were identified, the outcome of different approaches to compute the mean change was estimated and potential improvements are discussed to enhance policy support. Applying a model ensemble enabled us to obtain highly robust and consistent model results, substantially decreasing uncertainties in the scenario outcom
Recruitment Variability in North Atlantic Cod and Match-Mismatch Dynamics
Background
Fisheries exploitation, habitat destruction, and climate are important drivers of variability in recruitment success. Understanding variability in recruitment can reveal mechanisms behind widespread decline in the abundance of key species in marine and terrestrial ecosystems. For fish populations, the match-mismatch theory hypothesizes that successful recruitment is a function of the timing and duration of larval fish abundance and prey availability. However, the underlying mechanisms of match-mismatch dynamics and the factors driving spatial differences between high and low recruitment remain poorly understood.
Methodology/Principal Findings
We used empirical observations of larval fish abundance, a mechanistic individual-based model, and a reanalysis of ocean temperature data from 1960 to 2002 to estimate the survival of larval cod (Gadus morhua). From the model, we quantified how survival rates changed during the warmest and coldest years at four important cod spawning sites in the North Atlantic. The modeled difference in survival probability was not large for any given month between cold or warm years. However, the cumulative effect of higher growth rates and survival through the entire spawning season in warm years was substantial with 308%, 385%, 154%, and 175% increases in survival for Georges Bank, Iceland, North Sea, and Lofoten cod stocks, respectively. We also found that the importance of match-mismatch dynamics generally increased with latitude.
Conclusions/Significance
Our analyses indicate that a key factor for enhancing survival is the duration of the overlap between larval and prey abundance and not the actual timing of the peak abundance. During warm years, the duration of the overlap between larval fish and their prey is prolonged due to an early onset of the spring bloom. This prolonged season enhances cumulative growth and survival, leading to a greater number of large individuals with enhanced potential for survival to recruitment
Ecological commonalities among pelagic fishes: comparison of freshwater ciscoes and marine herring and sprat
Systematic comparisons of the ecology between functionally similar fish species from freshwater and marine aquatic systems are surprisingly rare. Here, we discuss commonalities and differences in evolutionary history, population genetics, reproduction and life history, ecological interactions, behavioural ecology and physiological ecology of temperate and Arctic freshwater coregonids (vendace and ciscoes, Coregonus spp.) and marine clupeids (herring, Clupea harengus, and sprat, Sprattus sprattus). We further elucidate potential effects of climate warming on these groups of fish based on the ecological features of coregonids and clupeids documented in the previous parts of the review. These freshwater and marine fishes share a surprisingly high number of similarities. Both groups are relatively short-lived, pelagic planktivorous fishes. The genetic differentiation of local populations is weak and seems to be in part correlated to an astonishing variability of spawning times. The discrete thermal window of each species influences habitat use, diel vertical migrations and supposedly also life history variations. Complex life cycles and preference for cool or cold water make all species vulnerable to the effects of global warming. It is suggested that future research on the functional interdependence between spawning time, life history characteristics, thermal windows and genetic differentiation may profit from a systematic comparison of the patterns found in either coregonids or clupeids
Low-frequency variability in North Sea and Baltic Sea identified through simulations with the 3-D coupled physical–biogeochemical model ECOSMO
Here we present results from a long-term model simulation
of the 3-D coupled ecosystem model ECOSMO II for a North Sea and Baltic Sea
set-up. The model allows both multi-decadal hindcast simulation of the marine
system and specific process studies under controlled environmental
conditions. Model results have been analysed with respect to long-term
multi-decadal variability in both physical and biological parameters with the
help of empirical orthogonal function (EOF) analysis. The analysis of a 61-year (1948–2008) hindcast reveals a quasi-decadal variation in
salinity, temperature and current fields in the North Sea in addition to
singular events of major changes during restricted time frames. These
changes in hydrodynamic variables were found to be associated with changes in
ecosystem productivity that are temporally aligned with the timing of
reported regime shifts in the areas. Our results clearly indicate that
for analysing ecosystem productivity, spatially explicit methods are
indispensable. Especially in the North Sea, a correlation analysis between
atmospheric forcing and primary production (PP) reveals significant
correlations between PP and the North Atlantic Oscillation (NAO) and wind forcing for the central part of the region,
while the Atlantic Multi-decadal Oscillation (AMO) and air temperature are correlated to long-term changes in PP in the
southern North Sea frontal areas. Since correlations cannot serve to
identify causal relationship, we performed scenario model runs
perturbing the temporal variability in forcing condition to emphasize
specifically the role of solar radiation, wind and eutrophication. The
results revealed that, although all parameters are relevant for the
magnitude of PP in the North Sea and Baltic Sea, the dominant impact on
long-term variability and major shifts in ecosystem productivity was
introduced by modulations of the wind fields
Towards a more complete understanding of the life cycle of brown shrimp (Crangon crangon): modelling passive larvae and juvenile transport in combination with physically forced vertical juvenile migration
In this study we developed and utilized a complex model approach to investigate the impact of stage-specific transport processes on the development and spatial distribution of brown shrimp (Crangon crangon) post-larvae and juveniles in the German Bight. First, we focused on drift processes during the pelagic larval stage by coupling an individual-based model for egg and larval development 'off-line' to a 3D hydrodynamic model utilizing the Lagrangian method. Secondly, we investigated tidal-induced transport processes after juvenile settlement. To determine the tidal cycle, the model coupling was accomplished 'on-line' by resolving the individual-based model and hydrodynamic model with the same time step. The vertical migration of juveniles, a prerequisite for the selective tidal stream transport (STST), was modelled as a sub-grid scale physical process (balance of forces: gravitation, buoyancy, Stoke's friction and dynamic uplift) and considered complex particle dynamics. We applied the model to test temperature and salinity cues as possible tidal indicators utilized by juvenile brown shrimp. Our results indicated that transport processes could significantly change the timing and spatial distribution of post-larval abundance. We also showed that the small-scale hydrodynamic forcing acting on the bodies of juvenile brown shrimps was sufficient to account for the vertical migration required to use STST. For both investigated tidal cues STST performing juvenile brown shrimp were transported on-shore. A faster and more continuous STST was calculated for the salinity cue, resulting in larger abundances of brown shrimp in estuarine areas