Climate-induced changes in the North Sea Decapoda over the last 60 years

In the mid-1980s the North Sea ecosystem experienced a climate-induced regime shift that has favoured decapods and detritivores in the benthos and jellyfish in the plankton over commercial fisheries. Here, we investigate changes among the Decapoda in the North Sea plankton over the last 60 yr. Decapods are important predators in the plankton and the benthos where they can influence productivity and structure communities. In the North Sea it has been suggested that a climate-driven increase in decapod abundance has been important in propagating the climate signal through the North Sea food web. We show that climate-induced changes in the Decapoda in the central and southern North Sea include the presence of new warm-water taxa, changes in the abundance and proportions of commercial species of shrimp, and an earlier occurrence of decapod larvae in the plankton compared with the period 1981–1983. Notable amongst the warm-water taxa appearing in the North Sea is the predatory swimming crab Polybius henslowii that can swarm in large numbers when conditions are favourable and that is known to exhibit range shifts in response to fluctuations in hydroclimatic forcing. We suggest that climate-induced changes among North Sea decapods have played an important role in the trophic amplification of a climate signal and the development of the new North Sea dynamic regime. Understanding these changes is likely to be imperative for a successful ecosystem-based approach to the future management of North Sea fisheries at a time of climate change.

Empirically modelling the potential effects of changes in temperature and prey availability on the growth of cod larvae in UK shelf seas

It has been hypothesized that changes in zooplankton community structure over the past four decades led to reduced growth and survival of prerecruit Atlantic cod (Gadus morhua) and that this was a key factor underlying poor year classes, contributing to stock collapse, and inhibiting the recovery of stocks around the UK. To evaluate whether observed changes in plankton abundance, species composition and temperature could have led to periods of poorer growth of cod larvae, we explored the effect of prey availability and temperature on early larval growth using an empirical trophodynamic model. Prey availability was parameterized using species abundance data from the Continuous Plankton Recorder. Our model suggests that the observed changes in plankton community structure in the North Sea may have had less impact on cod larval growth, at least for the first 40 days following hatching, than previously suggested. At least in the short term, environmental and prey conditions should be able to sustain growth of cod larvae and environmental changes acting on this early life stage should not limit stock recovery.

Have climate induced changes in zooplankton communities led to poor conditions for larval fish growth? A test on cod larvae on the UK shelf

Climate effects have been shown to be at least partly responsible for the reorganisation in the plankton ecosystem on the shelf seas of NW Europe over the last 50 years. Most fish larvae feed primarily on zooplankton, so changes in zooplankton quantity, quality and seasonal timing have been hypothesized to be a key factor affecting their survival. To investigate this we have implemented a 1-dimensional trophodynamic growth model of cod larvae for the waters around the UK covering the period 1960 to 2003. Larval growth is modelled as the difference between the amount of food absorbed by the larva and its various metabolic costs. Prey availability is based upon the biomass and size of available preys (i.e. adults and nauplii copepods and cladocerans) taken from the Continuous Plankton Recorder dataset. Temperature and wind forcing are also taken into account. Results suggest that observed changes in plankton community structure may have had less impact than previously suggested. This is because changes in prey availability may be compensated for by increased temperatures resulting in little overall impact on potential larval growth. Stock recovery, at least in the short term is likely to be more dependent upon conserving the year classes recruited to allow spawning stock biomass to rebuild. If as our model suggests, the larvae are still able to survive in the changing environment, reduction in fishing on the adults is needed to allow the stock to recover.

Region-wide changes in marine ecosystem dynamics: state-space models to distinguish trends from step changes

Regime shifts are sudden changes in ecosystem structure that can be detected across several ecosystem components. The concept that regime shifts are common in marine ecosystems has gained popularity in recent years. Many studies have searched for the step-like changes in ecosystem state expected under a simple interpretation of this idea. However, other kinds of change, such as pervasive trends, have often been ignored. We assembled over 300 ecological time series from seven UK marine regions, covering two to three decades. We developed state-space models for the first principal component of the time series in each region, a common measure of ecosystem state. Our models allowed both trends and step changes, possibly in combination. We found trends in three of seven regions and step changes in two of seven regions. Gradual and sudden changes are therefore important trajectories to consider in marine ecosystems.

Effects of increases in temperature and nutrients on phytoplankton community structure and photosynthesis in the Western English Channel

Anthropogenic climate change is exerting pressures on coastal ecosystems through increases in temperature, precipitation and ocean acidification. Phytoplankton community structure and photo-physiology are therefore adapting to these conditions. Changes in phytoplankton biomass and photosynthesis in relation to temperature and nutrient concentrations were assessed using a 14 year dataset from a coastal station in the Western English Channel (WEC). Dinoflagellate and coccolithophorid biomass exhibited a positive correlation with temperature, reaching the highest biomass at between 15 and 17°C. Diatoms showed a negative correlation with temperature, with highest biomass at 10°C. Chlorophyll a (chl a) normalised light-saturated photosynthetic rates (PBm) exhibited a hyperbolic response to increasing temperature, with an initial linear increase from 8 to 11°C, and reaching a plateau from 12°C. There was however no significant positive correlation between nutrients and phytoplankton biomass or PBm, which reflects the lag time between nutrient input and phytoplankton growth at this coastal site. The major phytoplankton groups that occurred at this site occupied distinct thermal niches, which in turn modified PBm. Increasing temperature, and higher water column stratification, was major factors in the initiation of dinoflagellates blooms at this site. Dinoflagellates blooms during summer also co-varied with silicate concentration, and acted as a tracer of dissolved inorganic nitrogen and phosphate from river run-off, which were subsequently reduced during these blooms. The data implies that increasing temperature and high river runoff during summer, will promote dinoflaglellates blooms in the WEC.

Temporal changes in total and size-fractioned chlorophyll-a in surface waters of three provinces in the Atlantic Ocean (September to November) between 2003 and 2010

Phytoplankton total chlorophyll concentration (TCHLa) and phytoplankton size structure are two important ecological indicators in biological oceanography. Using high performance liquid chromatography (HPLC) pigment data, collected from surface waters along the Atlantic Meridional Transect (AMT), we examine temporal changes in TCHLa and phytoplankton size class (PSC: micro-, nano- and pico-phytoplankton) between 2003 and 2010 (September to November cruises only), in three ecological provinces of the Atlantic Ocean. The HPLC data indicate no significant change in TCHLa in northern and equatorial provinces, and an increase in the southern province. These changes were not significantly different to changes in TCHLa derived using satellite ocean-colour data over the same study period. Despite no change in AMT TCHLa in northern and equatorial provinces, significant differences in PSC were observed, related to changes in key diagnostic pigments (fucoxanthin, peridinin, 19′-hexanoyloxyfucoxanthin and zeaxanthin), with an increase in small cells (nano- and pico-phytoplankton) and a decrease in larger cells (micro-phytoplankton). When fitting a three-component model of phytoplankton size structure — designed to quantify the relationship between PSC and TCHLa to each AMT cruise, model parameters varied over the study period. Changes in the relationship between PSC and TCHLa have wide implications in ecology and marine biogeochemistry, and provide key information for the development and use of empirical ocean-colour algorithms. Results illustrate the importance of maintaining a time-series of in-situ observations in remote regions of the ocean, such as that acquired in the AMT programme.

Impact of sub-seabed CO2 leakage on macrobenthic community structure and diversity

A sub-seabed release of carbon dioxide (CO2) was conducted to assess the potential impacts of leakage from sub-seabed geological CO2 Capture and Storage CCS) on benthic macrofauna. CO2 gas was released 12 m below the seabed for 37 days, causing significant disruption to sediment carbonate chemistry. Regular macrofauna samples were collected from within the area of active CO2 leakage (Zone 1) and in three additional reference areas, 25 m, 75 m and 450 m from the centre of the leakage (Zones 2, 3 and 4 respectively). Macrofaunal community structure changed significantly in all zones during the study period. However, only the changes in Zone 1 were driven by the CO2 leakage with the changes in reference zones appearing to reflect natural seasonal succession and stochastic weather events. The impacts in Zone 1 occurred rapidly (within a few days), increased in severity through the duration of the leak, and continued to worsen after the leak had stopped. Considerable macrofaunal recovery was seen 18 days after the CO2 gas injection had stopped. In summary, small short-term CCS leakage events are likely to cause highly localised impacts on macrofaunal communities and there is the potential for rapid recovery to occur, depending on the characteristics of the communities and habitats impacted.

Can benthic community structure be used to predict the process of bioturbation in real ecosystems

Disentangling the roles of environmental change and natural environmental variability on biologically mediated ecosystem processes is paramount to predict future marine ecosystem functioning. Bioturbation, the biogenic mixing of sediments, has a regulating role in marine biogeochemical processes. However, our understanding of bioturbation as a community level process and of its environmental drivers is still limited by loose use of terminology, and a lack of consensus about what bioturbation is. To help resolve these challenges, this empirical study investigated the links between four different attributes of bioturbation (bioturbation depth, activity and distance, and biodiffusive transport); the ability of an index of bioturbation (BPc) to predict each of them; and their relation to seasonality, in a shallow coastal system – the Western Channel Observatory, UK. Bioturbation distance depended on changes in benthic community structure, while the other three attributes were more directly influenced by seasonality in food availability. In parallel, BPc successfully predicted bioturbation distance but not the other attributes of bioturbation. This study therefore highlights that community bioturbation results from this combination of processes responding to environmental variability at different time-scales. However, community level measurements of bioturbation across environmental variability are still scarce, and BPc is calculated using commonly available data on benthic community structure and the functional classification of invertebrates. Therefore, BPc could be used to support the growth of landscape scale bioturbation research, but future uses of the index need to consider which bioturbation attributes the index actually predicts. As BPc predicts bioturbation distance, estimated here using a random-walk model applicable to community settings, studies using either of the metrics should be directly comparable and contribute to a more integrated future for bioturbation research.

Disentangling the impacts of heat wave magnitude, duration and timing on the structure and diversity of sessile marine assemblages

Extreme climatic events, including heat waves (HWs) and severe storms, influence the structure of marine and terrestrial ecosystems. Despite growing consensus that anthropogenic climate change will increase the frequency, duration and magnitude of extreme events, current understanding of their impact on communities and ecosystems is limited. Here, we used sessile invertebrates on settlement panels as model assemblages to examine the influence of HW magnitude, duration and timing on marine biodiversity patterns. Settlement panels were deployed in a marina in southwest UK for ≥5 weeks, to allow sufficient time for colonisation and development of sessile fauna, before being subjected to simulated HWs in a mesocosm facility. Replicate panel assemblages were held at ambient sea temperature (∼17 °C), or +3 °C or +5 °C for a period of 1 or 2 weeks, before being returned to the marina for a recovery phase of 2–3 weeks. The 10-week experiment was repeated 3 times, staggered throughout summer, to examine the influence of HW timing on community impacts. Contrary to our expectations, the warming events had no clear, consistent impacts on the abundance of species or the structure of sessile assemblages. With the exception of 1 high-magnitude long-duration HW event, warming did not alter not assemblage structure, favour non-native species, nor lead to changes in richness, abundance or biomass of sessile faunal assemblages. The observed lack of effect may have been caused by a combination of (1) the use of relatively low magnitude, realistic heat wave treatments compared to previous studies (2), the greater resilience of mature adult sessile fauna compared to recruits and juveniles, and (3) the high thermal tolerance of the model organisms (i.e., temperate fouling species, principally bryozoans and ascidians). Our study demonstrates the importance of using realistic treatments when manipulating climate change variables, and also suggests that biogeographical context may influence community-level responses to short-term warming events, which are predicted to increase in severity in the future.

A tale of two seas: contrasting patterns of population structure in the small-spotted catshark across Europe.

Elasmobranchs represent important components of marine ecosystems, but they can be vulnerable to overexploitation. This has driven investigations into the population genetic structure of large-bodied pelagic sharks, but relatively little is known of population structure in smaller demersal taxa, which are perhaps more representative of the biodiversity of the group. This study explores spatial population genetic structure of the small-spotted catshark (Scyliorhinus canicula), across European seas. The results show significant genetic differences among most of the Mediterranean sample collections, but no significant structure among Atlantic shelf areas. The data suggest the Mediterranean populations are likely to have persisted in a stable and structured environment during Pleistocene sea-level changes. Conversely, the Northeast Atlantic populations would have experienced major changes in habitat availability during glacial cycles, driving patterns of population reduction and expansion. The data also provide evidence of male-biased dispersal and female philopatry over large spatial scales, implying complex sex-determined differences in the behaviour of elasmobranchs. On the basis of this evidence, we suggest that patterns of connectivity are determined by trends of past habitat stability that provides opportunity for local adaptation in species exhibiting philopatric behaviour, implying that resilience of populations to fisheries and other stressors may differ across the range of species.

An unusually large phytoplankton spring bloom drives rapid changes in benthic diversity and ecosystem function

In 2012, the Western English Channel experienced an unusually large and long-lived phytoplankton spring bloom. When compared with data from the past 20 years, average phytoplankton biomass at Station L4 (part of the Western Channel Observatory) was approximately 3× greater and lasted 50% longer than any previous year. Regular (mostly weekly) box core samples were collected from this site before, during and after the bloom to determine its impact on macrofaunal abundance, diversity, biomass, community structure and function. The spring bloom of 2012 was shown to support a large and rapid response in the majority of benthic taxa and functional groups. However, key differences in the precise nature of this response, as well as in its timing, was observed between different macrofauna feeding groups. Deposit feeders responded almost instantly at the start of the bloom, primarily thorough an increase in abundance. Suspension feeders and opportunistic/predatory/carnivorous taxa responded slightly more slowly and primarily with an increase in biomass. At the end of the bloom a rapid decline in macrobenthic abundance, diversity and biomass closely followed the decline in phytoplankton biomass. With suspension feeders showing evidence of this decline a few weeks before deposit feeders, it was concluded that this collapse in benthic communities was driven primarily by food availability and competition. However, it is possible that environmental hypoxia and the presence of toxic benthic cyanobacteria could also have contributed to this decline. This study shows evidence for strong benthic–pelagic coupling at L4; a shallow (50 m), coastal, fine-sand habitat. It also demonstrates that in such habitats, it is not just planktonic organisms that demonstrate clear community phenology. Different functional groups within the benthic assemblage will respond to the spring bloom in specific manner, with implications for key ecosystem functions and processes, such as secondary production and bioturbation. Only by taking integrated benthic and pelagic observations over such fine temporal scales (weekly) was the current study able to identify the intimate structure of the benthic response. Similar studies from other habitats and under different bloom conditions are urgently needed to fully appreciate the strength of benthic–pelagic coupling in shallow coastal environments.