Large-scale study of Calanus in the North Atlantic Ocean: macroecological patterns and potential impacts of climate change

Marine ecosystems show natural fluctuation throughout a large range of spatial and temporal scales. Despite the large amount of study devoted to the North Atlantic Ocean, drivers of those fluctuations remain unclear. By changing global climate, polluting, introducing exotic species, expanding and intensifying land uses and overharvesting biological resources, human activities have degraded the global ecosystem and drastically accelerated species extinction rates. Consequences of this human forcing become apparent in the progressive degradation of ecosystem that are used by humans (Schroter et al. , 2005), climate change- induced shifts in species distributions toward the poles (Parmesan et al. , 1999) and higher elevations (Wilson et al. , 2005), and in rapidly changing phenology (Edwards & Richardson, 2004). Data collected by the Continuous Plankton Recorder (CPR) constitutes, by both their temporal and biogeographical extends, one of the most useful datasets to investigate further major marine management issues as the distinction between anthropogenic, climatically forced and natural ecosystems fluctuations. The present work is a contribution to environmental change biology focused on copepods Calanus species as key structural species characteristic of the North Atlantic Ocean and adjacent seas. The purpose is to (1) identify environmental factors leading to the large-scale distribution patterns of Calanus that occurred in the North Atlantic Ocean, and (2) to propose and investigate new methods to assess both fundamental and realised niches of a dominant species in these basins. Most current approaches using Hutchinson concept of ecological niches to model species distribution belong to correlative or mechanistic models. A correlative approach has been developed to assess statistical relationships between the observed spatial distributions of two congeneric species and a set of environmental variables characteristic of the studied area. The method is designed to show the seasonal dynamics of environmental restriction driving observed distributions. Both Calanus finmarchicus and C. helgolandicus environmental preferences and optimum have been defined for 11 environmental parameters. A principal component analysis (PCA) has been used (1) to quantify the importance on the spatial distribution of each environmental parameter and (2) to identify the ecological niche. A numerical analysis based on Multiple Response Permutation Procedures (MRPP) was utilised to assess the breath of each niche and to compare them. The egg production rate of Calanus finmarchicus has been defined to investigate the link between physiology, macroecological patterns and ecological niches. It typically assesses the fundamental niche as in opposition to the correlative approach, the model based on a fundamental biological process is more focused on the potential response of C. finmarchicus to environmental conditions. The simplicity of the method which used only Sea Surface Temperature (SST) allows us to use IPCC scenarios and predict a shift in distribution over the 21st century.Sir Alister Hardy Foundation for Ocean Science and CNRS Franc

Sensitivity of Calanus spp. copepods to environmental changes in the North Sea using life-stage structured models

The copepods Calanus finmarchicus and C. helgolandicus co-exist in the North Sea, but their spatial distribution and phenology are very different. Long-term changes in their distributions seem to occur due to climate change resulting in a northward extension of C. helgolandicus and a decline of C. finmarchicus in this region. The aim of this study is to use life-stage structured models of the two Calanus species embedded in a 3D coupled hydrodynamic-biogeochemical model to investigate how the biogeography of C. finmarchicus and C. helgolandicus is modified by changes in ± 2°C sea water temperatures, overwintering and oceanic inflow in the North Sea. Life-stage structured models are validated against CPR data and vertical distributions north of the Dogger Bank in the North Sea for the reference year 2005. The model shows that 1) ± 2°C changes from the current level mainly influence the seasonal patterns and not the relative occurrence of the two species, 2) changes due to oceanic inflow mainly appeared in the northern and southern part of the North Sea connected to the NE Atlantic and not in the central part and 3) the abundance of Calanus species were very sensitive to the degree of overwintering within the North Sea because it allows them to utilize the spring bloom more efficiently and independently of the timing and amount of oceanic inflow. The combination of lower temperatures, higher overwintering and oceanic inflow simulating the situation in the 1960s largely favoured C. finmarchicus and their relative contribution to Calanus spp. increased from 40% in the reference year to 72%. The +2°C scenario suggest that in a warmer future, C. finmarchicus is likely to decline and C. helgolandicus abundance will probably continue to increase in some areas

Temporal Variability of Co-Occurring Calanus finmarchicus and C. helgolandicus in Skagerrak

The congeneric copepods Calanus finmarchicus and C. helgolandicus co-occur in the northern North Sea and Skagerrak where they play important roles as prey for higher trophic levels. This study analyses a 26-year time series (1994-2019) with a ~ two-week temporal resolution for Calanus spp. at a fixed monitoring station in Skagerrak, off southern Norway. Seasonal variation, inter-annual variability and long-term trends for the two species were examined. Strong differences in the species-specific seasonality were revealed, with C. finmarchicus dominating in spring and C. helgolandicus in autumn. The seasonal peak of C. finmarchicus was associated with relatively low temperatures (6-8°C) and high chlorophyll a concentration, while C. helgolandicus displayed its seasonal maximum at higher temperatures (11-16°C). C. finmarchicus was found to produce one dominant annual generation (in spring), but two or more generations are considered likely. Contrasting long-term trends in abundances were found for the two species, suggesting that their population sizes were affected by different mechanisms. The abundance of C. helgolandicus showed an increasing trend over the time period studied, apart from the last years. The abundance of C. finmarchicus was more variable and displayed no unidirectional long-term trends over the time series. The study revealed a shift in the phenology of Calanus spp. over the last 25 years at this site. Since 1994 the timing of the annual peaks in both C. finmarchicus and C. helgolandicus have advanced by about one month. Moreover, the seasonal pattern of C. helgolandicus, switched from a unimodal to a bimodal pattern around 2002, with a small additional peak also appearing in spring. The results suggest that the proximity to the Norwegian Trench influences the demography and abundance of C. finmarchicus in this coastal area, both as a gateway for the advective supply, as well as a habitat for local overwintering.publishedVersio

Spatial modelling of Calanus finmarchicus and Calanus helgolandicus : parameter differences explain differences in biogeography

The North Atlantic copepods Calanus finmarchicus and C. helgolandicus are moving north in response to rising temperatures. Understanding the drivers of their relative geographic distributions is required in order to anticipate future changes. To explore this, we created a new spatially explicit stage-structured model of their populations throughout the North Atlantic. Recent advances in understanding Calanus biology, including U-shaped relationships between growth and fecundity and temperature, and a new model of diapause duration are incorporated in the model. Equations were identical for both species, but some parameters were species-specific. The model was parameterized using Continuous Plankton Recorder Survey data and tested using time series of abundance and fecundity. The geographic distributions of both species were reproduced by assuming that only known interspecific differences and a difference in the temperature influence on mortality exist. We show that differences in diapause capability are not necessary to explain why C. helgolandicus is restricted to the continental shelf. Smaller body size and higher overwinter temperatures likely make true diapause implausible for C. helgolandicus. Known differences were incapable of explaining why only C. helgolandicus exists southwest of the British Isles. Further, the fecundity of C. helgolandicus in the English Channel is much lower than we predict. We hypothesize that food quality is a key influence on the population dynamics of these species. The modeling framework presented can potentially be extended to further Calanus species

Seasonal Phytoplankton Blooms in the North Atlantic Linked to the Overwintering Strategies of Copepods

The North Atlantic Ocean contains diverse patterns of seasonal phytoplankton blooms with distinct internal dynamics. We analyzed blooms using remotely-sensed chlorophyll a concentration data and change point statistics. The first bloom of the year began during spring at low latitudes and later in summer at higher latitudes. In regions where spring blooms occurred at high frequency (i. e., proportion of years that a bloom was detected), there was a negative correlation between bloom timing and duration, indicating that early blooms last longer. In much of the Northeast Atlantic, bloom development extended over multiple seasons resulting in peak chlorophyll concentrations in summer. Spring bloom start day was found to be positively correlated with a spring phenology index and showed both positive and negative correlations to sea surface temperature and the North Atlantic Oscillation in different regions. Based on the characteristics of spring and summer blooms, the North Atlantic can be classified into two regions: a seasonal bloom region, with a well-defined bloom limited to a single season; and a multi-seasonal bloom region, with blooms extending over multiple seasons. These regions differed in the correlation between bloom start and duration with only the seasonal bloom region showing a significant, negative correlation. We tested the hypothesis that the near-surface springtime distribution of copepods that undergo diapause (Calanus finmarchicus, C. helgolandicus, C. glacialis, and C. hyperboreus) may contribute to the contrast in bloom development between the two regions. Peak near-surface spring abundance of the late stages of these Calanoid copepods was generally associated with areas having a well-defined seasonal bloom, implying a link between bloom shape and their abundance. We suggest that either grazing is a factor in shaping the seasonal bloom or bloom shape determines whether a habitat is conducive to diapause, while recognizing that both factors can re-enforce each other

Spatio-temporal variation in the zooplankton prey of lesser sandeels : species and community trait patterns from the continuous plankton recorder

The phenology, distribution and size composition of plankton communities are changing rapidly in response to warming. This may lead to shifts in the prey fields of planktivorous fish, which play a key role in transferring energy up marine food chains. Here, we use 60+ years of Continuous Plankton Recorder data to explore temporal trends in key taxa and community traits in the prey field of planktivorous lesser sandeels (Ammodytes marinus) in the North Sea, the Faroes and southern Iceland. We found marked spatial variation in the prey field, with Calanus copepods generally being a key component much more common in the northern part of the study area, but not further south. In the western North Sea, the estimated amount of available energy in the prey field has decreased by more than 50% since the 1960s. This decrease was accompanied by declining abundances of small copepods, and shifts in the timing of peak annual prey abundances out of the sandeel foraging season. Further, the estimated average prey community body prey size has increased in several of the locations considered. Overall, our results point to the importance of considering the full prey field of planktivores regional studies of prey fields, rather than and caution against inferring ecological consequences based only on large-scale trends in key taxa or mean community traits

Seasonality and interannual variability of copepods in the Western English Channel, Celtic Sea, Bay of Biscay, and Cantabrian Sea with a special emphasis to Calanus helgolandicus and Acartia clausi

A total of five mesozooplankton time series data sets were assembled to compare the seasonal and interannual patterns of abundance of calanoid copepods in the Western English Channel (Station L4), Celtic Sea, Bay of Biscay (Continuous Plankton Recorder), and the Cantabrian Sea (RADIALES time series, Santander, St-4 and St-6) from January 1992 to December 1999. A strong seasonal component in taxonomic composition was detected at the locations considered. There was also a strong latitudinal effect on diversity at each location, southernmost locations being more diverse. The seasonal dynamics and year-to-year variability of two copepod species: Calanus helgolandicus and Acartia clausi were studied in detail. A latitudinal pattern in the seasonal cycles of both copepod species was observed. The peaks of both occur earlier in spring in the warmer southern region and move northwards, consistent with the temperature regimes at each location, supporting the broad concept that species occupy a thermal niche in time as well as in space. There was a strong degree of interannual variability between sites and between species. No clear trends, but some coherent events among data sets, reveal a regional response to environmental forcing factors. Correlations suggest possible connections with environmental indices like the North Atlantic Oscillation and the Gulf Stream North Wall index. There was a positive correlation between the NAO and the abundance of C. helgolandicus at station L4 off Plymouth; however, the relationship in the Celtic Sea and Bay of Biscay was opposite to that expected based on previous results. Despite the differences in the sampling techniques used within each dataset, the results are comparable and coherent in terms of taxonomic composition and the seasonal and interannual patterns detected

Response to changing oceanography in the Dove Time Series : a Northumberland plankton community study

PhD ThesisThe Dove Time Series is a plankton monitoring station in the Northumberland coastal sea which has been sampled since 1969. Over the 20th century, major changes have occurred in the North Sea plankton which have in part correlated with oscillation in atmospheric mass over the Northern Hemisphere, the North Atlantic Oscillation (NAO). Westerly winds when the NAO is positive phase block northern low pressure systems and transport warm Atlantic water into the North Sea extending stratification, leading to greater phytoplankton biomass. Phytoplankton biomass in the central North Sea reached a sustained higher level after 1985. Phytoplankton, zooplankton and ichthyoplankton datasets were created or extended from the Dove Time Series to study the effect of oceanographic change at this location. There was a change to a high abundance community 10 years later, in 1995. The most important predictor of phytoplankton abundance was not the NAO index, but the Atlantic Meridional Oscillation (AMO), which exhibits 60-100 year and subordinate 11 and 14 year periodicity, describing a deviation from the long term sea surface temperature (SST) mean in the North Atlantic. Phytoplankton periodicity partly matched the 14 year period in the AMO, which correlates with a feedback mechanism of westerly versus northerly wind in the North Atlantic, regulating ocean-atmosphere heat flux. Zooplankton abundance was predicted by SST and ratio of maximum to minimum abundance by phytoplankton/AMO. Oceanographic conditions that were contemporary with the state of the AMO anomaly after 1995 promoted higher spring phytoplankton abundance and neritic copepod abundance peaks. Ichthyoplankton variability was not synchronous with these lower order changes, probably as a result of different effects on adult fish. The cyclical nature of the AMO means both low and high biomass communities observed in the Dove Time Series are part of one regime.Natural Environment Research Counci

Seasonal Phytoplankton Blooms in the North Atlantic Linked to the Overwintering Strategies of Copepods

The North Atlantic Ocean contains diverse patterns of seasonal phytoplankton blooms with distinct internal dynamics. We analyzed blooms using remotely-sensed chlorophyll a concentration data and change point statistics. The first bloom of the year began during spring at low latitudes and later in summer at higher latitudes. In regions where spring blooms occurred at high frequency (i. e., proportion of years that a bloom was detected), there was a negative correlation between bloom timing and duration, indicating that early blooms last longer. In much of the Northeast Atlantic, bloom development extended over multiple seasons resulting in peak chlorophyll concentrations in summer. Spring bloom start day was found to be positively correlated with a spring phenology index and showed both positive and negative correlations to sea surface temperature and the North Atlantic Oscillation in different regions. Based on the characteristics of spring and summer blooms, the North Atlantic can be classified into two regions: a seasonal bloom region, with a well-defined bloom limited to a single season; and a multi-seasonal bloom region, with blooms extending over multiple seasons. These regions differed in the correlation between bloom start and duration with only the seasonal bloom region showing a significant, negative correlation. We tested the hypothesis that the near-surface springtime distribution of copepods that undergo diapause (Calanus finmarchicus, C. helgolandicus, C. glacialis, and C. hyperboreus) may contribute to the contrast in bloom development between the two regions. Peak near-surface spring abundance of the late stages of these Calanoid copepods was generally associated with areas having a well-defined seasonal bloom, implying a link between bloom shape and their abundance. We suggest that either grazing is a factor in shaping the seasonal bloom or bloom shape determines whether a habitat is conducive to diapause, while recognizing that both factors can re-enforce each other