Population-Growth And Vertical-Distribution Of Calanus-Helgolandicus In The Celtic Sea

Calanus helgolandicus over-winters in the shallow waters (100 m) of the Celtic Sea as copepodite stages V and VI; the minimum temperature in winter is approximately 8.0°C. This over-wintering is not a true hibernation or dormacy, accompanied by a reduced metabolic state with a discontinuation of feeding and development, but more of a lowered activity, involving reduced feeding and development, with predation on available microzooplankton and detritus. Analysis of specimens from the winter population showed that copepodite stages V and VI were actively feeding and still producing and possibly liberating eggs. The absence of late nauplii and young copepodites in the water column until late March indicated that there must be a high mortality of these winter cohorts. The copepodites of the first generation appeared in April–May, the younger stages, copepodites I to III, being distributed deeper in the water column below the euphotic zone and thermocline. This distribution would contribute to amuch slower rate of development. By August the ontogenetic vertical distributions observed in the copepodites were reversed, the younger stages occuring in the warmer surface layers within the euphotic zone. Diurnal migrations were observed in the later copepodites only, the younger stages I to III either remaining deep in spring or shallow in summer. The causal mechanisms which alter the behaviour of the young copepodites remain unexplained. The development of the population of Calanus helgolandicus in 1978, reaching its peak of abundance in August, was typical for the shelf seas around U.K. as observed from Continuous Plankton Recorder data, 1958 to 1977.

The Influence Of Temperature And Salinity On Energy Partitioning In The Marine Nematode Diplolaimelloides-Bruciei

Measurements of population growth, generation time, fecundity and respiration in laboratory culture have been made, in relation to temperature and salinity, for the nematode Diplolaimelloides bruciei Hopper, a species normally associated with decayed material of the marsh grass Spartina. The intrinsic rate of increase (r) is high: it is related to temperature between 5° and 25°C by a sigmoid function which is steepest between 10° and 15°C, and is maximum at 26‰ salinity. Generation time is related to temperature by a power function and is shortest at 26‰ salinity. The effect of temperature on generation time is consistent with other data for marine nematodes, and the steep slope of r against temperature is largely due to the marked effect of temperature on fecundity. A sex ratio of 2:1 in favour of males is maintained regardless of culture conditions or population density. Respiration increases exponentially with temperature between 5° and 25°C, with a very high Q10 (3.94), but is not affected by salinity. At 30°C respiration is no higher than at 25°C. A high and relatively stable production efficiency (P/A) is maintained between 10 and 30°C with a maximum of 87% at 15°C; there is a stable reproductive effort (Pr/A) of about 10%. At 5°C both these ratios are zero. Data for the harpacticoid copepod Tachidius discipes, derived from the literature, show that this too has a high and stable production efficiency, which may be a characteristic of meiofaunal species in general, but in this species efficiency is relatively high at 5°C. Many features of the energy balance in D. bruciei can be related to an opportunistic mode of life.

Influence of climate change and trophic coupling across four trophic levels in the Celtic Sea

Climate change has had profound effects upon marine ecosystems, impacting across all trophic levels from plankton to apex predators. Determining the impacts of climate change on marine ecosystems requires understanding the direct effects on all trophic levels as well as indirect effects mediated by trophic coupling. The aim of this study was to investigate the effects of climate change on the pelagic food web in the Celtic Sea, a productive shelf region in the Northeast Atlantic. Using long-term data, we examined possible direct and indirect ‘bottom-up’ climate effects across four trophic levels: phytoplankton, zooplankton, mid-trophic level fish and seabirds. During the period 1986–2007, although there was no temporal trend in the North Atlantic Oscillation index (NAO), the decadal mean Sea Surface Temperature (SST) in the Celtic Sea increased by 0.66±0.02°C. Despite this, there was only a weak signal of climate change in the Celtic Sea food web. Changes in plankton community structure were found, however this was not related to SST or NAO. A negative relationship occurred between herring abundance (0- and 1-group) and spring SST (0-group: p = 0.02, slope = −0.305±0.125; 1-group: p = 0.04, slope = −0.410±0.193). Seabird demographics showed complex species–specific responses. There was evidence of direct effects of spring NAO (on black-legged kittiwake population growth rate: p = 0.03, slope = 0.0314±0.014) as well as indirect bottom-up effects of lagged spring SST (on razorbill breeding success: p = 0.01, slope = −0.144±0.05). Negative relationships between breeding success and population growth rate of razorbills and common guillemots may be explained by interactions between mid-trophic level fish. Our findings show that the impacts of climate change on the Celtic Sea ecosystem is not as marked as in nearby regions (e.g. the North Sea), emphasizing the need for more research at regional scales.

Have we been underestimating the effects of ocean acidification in zooplankton?

Understanding how copepods may respond to ocean acidification (OA) is critical for risk assessments of ocean ecology and biogeochemistry. The perception that copepods are insensitive to OA is largely based on experiments with adult females. Their apparent resilience to increased carbon dioxide (pCO2 ) concentrations has supported the view that copepods are 'winners' under OA. Here, we show that this conclusion is not robust, that sensitivity across different life stages is significantly misrepresented by studies solely using adult females. Stage-specific responses to pCO2 (385-6000 μatm) were studied across different life stages of a calanoid copepod, monitoring for lethal and sublethal responses. Mortality rates varied significantly across the different life stages, with nauplii showing the highest lethal effects; nauplii mortality rates increased threefold when pCO2 concentrations reached 1000 μatm (year 2100 scenario) with LC50 at 1084 μatm pCO2 . In comparison, eggs, early copepodite stages, and adult males and females were not affected lethally until pCO2 concentrations ≥3000 μatm. Adverse effects on reproduction were found, with >35% decline in nauplii recruitment at 1000 μatm pCO2 . This suppression of reproductive scope, coupled with the decreased survival of early stage progeny at this pCO2 concentration, has clear potential to damage population growth dynamics in this species. The disparity in responses seen across the different developmental stages emphasizes the need for a holistic life-cycle approach to make species-level projections to climate change. Significant misrepresentation and error propagation can develop from studies which attempt to project outcomes to future OA conditions solely based on single life history stage exposures.

The future of the northeast Atlantic benthic flora in a high CO2 world

Seaweed and seagrass communities in the northeast Atlantic have been profoundly impacted by humans, and the rate of change is accelerating rapidly due to runaway CO2 emissions and mounting pressures on coastlines associated with human population growth and increased consumption of finite resources. Here, we predict how rapid warming and acidification are likely to affect benthic flora and coastal ecosystems of the northeast Atlantic in this century, based on global evidence from the literature as interpreted by the collective knowledge of the authorship. We predict that warming will kill off kelp forests in the south and that ocean acidification will remove maerl habitat in the north. Seagrasses will proliferate, and associated epiphytes switch from calcified algae to diatoms and filamentous species. Invasive species will thrive in niches liberated by loss of native species and spread via exponential development of artificial marine structures. Combined impacts of seawater warming, ocean acidification, and increased storminess may replace structurally diverse seaweed canopies, with associated calcified and noncalcified flora, with simple habitats dominated by noncalcified, turf-forming seaweeds.

Long-term variability of the siphonophores Muggiaea atlantica and M. kochi in the Western English Channel

We investigated long-term variability of the calycophoran siphonophores Muggiaea atlantica and Muggiaea kochi in the Western English Channel (WEC) between 1930 and 2011. Our aims were to describe long-term changes in abundance and temporal distribution in relation to local environmental dynamics. In order to better understand mechanisms that regulate the species’ populations, we identified periods that were characteristic of in situ population growth and the environmental optima associated with these events. Our results show that between 1930 and the 1960s both M. atlantica and M. kochi were transient components of the WEC ecosystem. In the late 1960s M. atlantica, successfully established a resident population in the WEC, while the occurrence of M. kochi became increasingly sporadic. Once established as a resident species, the seasonal abundance and distribution of M. atlantica increased. Analysis of environmental conditions associated with in situ population growth revealed that temperature and prey were key determinants of the seasonal distribution and abundance of M. atlantica. Salinity was shown to have an indirect effect, likely representing a proxy for water circulation in the WEC. Anomalies in the seasonal cycle of salinity, indicating deviation from the usual circulation pattern in the WEC, were negatively associated with in situ growth, suggesting dispersal of the locally developing M. atlantica population. However, our findings identified complexity in the relationship between characteristics of the environment and M. atlantica variability. The transition from a period of transiency (1930–1968) to residency (1969–2011) was tentatively attributed to structural changes in the WEC ecosystem that occurred under the forcing of wider-scale hydroclimatic changes.