The impact of ocean acidification and warming on the skeletal mechanical properties of the sea urchin Paracentrotus lividus from laboratory and field observations

Increased atmospheric CO2 concentration is leading to changes in the carbonate chemistry and the temperature of the ocean. The impact of these processes on marine organisms will depend on their ability to cope with those changes, particularly the maintenance of calcium carbonate structures. Both a laboratory experiment (long-term exposure to decreased pH and increased temperature) and collections of individuals from natural environments characterized by low pH levels (individuals from intertidal pools and around a CO2 seep) were here coupled to comprehensively study the impact of near-future conditions of pH and temperature on the mechanical properties of the skeleton of the euechinoid sea urchin Paracentrotus lividus. To assess skeletal mechanical properties, we characterized the fracture force, Young's modulus, second moment of area, material nanohardness, and specific Young's modulus of sea urchin test plates. None of these parameters were significantly affected by low pH and/or increased temperature in the laboratory experiment and by low pH only in the individuals chronically exposed to lowered pH from the CO2 seeps. In tidal pools, the fracture force was higher and the Young's modulus lower in ambital plates of individuals from the rock pool characterized by the largest pH variations but also a dominance of calcifying algae, which might explain some of the variation. Thus, decreases of pH to levels expected for 2100 did not directly alter the mechanical properties of the test of P. lividus. Since the maintenance of test integrity is a question of survival for sea urchins and since weakened tests would increase the sea urchins' risk of predation, our findings indicate that the decreasing seawater pH and increasing seawater temperature expected for the end of the century should not represent an immediate threat to sea urchins vulnerability.

Use of satellite data for modelling food availability and survival of marine fish larvae

A modelling scheme is described which uses satellite retrieved sea-surface temperature and chlorophyll-a to derive monthly zooplankton biomass estimates in the eastern North Atlantic; this forms part of a bio-physical model of inter-annual variations in the growth and survival of larvae and post-larvae of mackerel (Scomber scombrus). The temperature and chlorophyll data are incorporated first to model copepod (Calanus) egg production rates. Egg production is then converted to available food using distribution data from the Continuous Plankton Recorder (CPR) Survey, observed population biomass per unit daily egg production and the proportion of the larval mackerel diet comprising Calanus. Results are validated in comparison with field observations of zooplankton biomass. The principal benefit of the modelling scheme is the ability to use the combination of broad scale coverage and fine scale temporal and spatial variability of satellite data as driving forces in the model; weaknesses are the simplicity of the egg production model and the broad-scale generalizations assumed in the raising factors to convert egg production to biomass.

Wind mixing, food availability and mortality of anchovy larvae Engraulis encrasicolus in the northern Adriatic Sea

A study was carried out in June/July 1996 in the River Po outflow in the northern Adriatic to investigate spawning of anchovy Engraulis encrasicolus and survival of larvae in relation to food availability and wind mixing. Hydrographic- and bongo net sampling was carried out on 2 grid surveys; one after a period of low winds and settled weather, and the other after an intervening period of strong winds, which resulted in a decrease in water column stratification. The spawning areas of anchovy and the larval distributions were associated with the river outflow plume (most clearly on the second survey grid, after the period of higher winds). Potential food particles for anchovy larvae, primarily copepod nauplii and copepodite stages, were also concentrated in the area influenced by the river outflow. Although there was a nearly 50% reduction in the mean water column abundance of potential food particles between the 2 survey grids, mostly due to a decline in abundance outside the immediate river plume area, there was no significant change in mortality of anchovy larvae between the 2 grids; the exponential decline in numbers of eggs and larvae to 10 mm in length being equivalent to overall mortality rates of 43.2%/d on the first survey and 44.7%/d on the second. The resilience of larval survival under potentially less favourable feeding conditions, following the period of wind mixing, was ascribed, in part, to the maintenance of local water column stratification by the superficial low salinity input from the River Po. This stratification in the immediate outflow area was associated with the presence of concentrated layers of potential food particles (typically >50 particles/L and 1.5 to 2.8 times the mean water column abundance) in the upper 10 m of the water column, coincident with peak numbers of anchovy larvae. However, since there was no evidence for lower larval survival in areas, less influenced by the immediate river outflow plume, a simple direct relationship between enhanced water column stability, improved feeding conditions and larval survival was not supported.

Climate effects and benthic-pelagic coupling in the North Sea

The North Sea is one of the most biologically productive ecosystems in the world and supports important fisheries. Climate-induced changes occurred in the pelagic ecosystems of the North Sea during the 1980s. These changes, which have been observed from phytoplankton to fish and among permanent (holoplankton) and temporary (meroplankton) plankton species, have resulted in alterations in plankton community composition and seasonality. Until now, the effects of climate-driven changes on biological linkages between pelagic and benthic ecosystems have not been examined. The present study indicates that changes in benthic organisms could have a profound effect on the trophodynamics of the pelagos. We demonstrate this by analyses of a long-term time series of North Sea plankton and sea surface temperature data. We discover that pronounced changes in the North Sea meroplankton, mainly related to an increased abundance and spatial distribution of the larvae of a benthic echinoderm, Echinocardium cordatum, result primarily from a stepwise increase in sea temperature after 1987 that has caused warmer conditions to occur earlier in the year than previously. Key stages of reproduction in E. cordatum, gametogenesis and spawning, appear to be influenced by winter and spring sea temperature and their larval development is affected by the quantity and quality of their phytoplankton food. Our analyses suggest that a new thermal regime in the North Sea in winter and spring may have benefited reproduction and survival in this benthic species. As a result, E. cordatum may be altering the trophodynamics of the summer pelagic ecosystem through competition between its larvae and holozooplankton taxa.

Reduced pH sea water disrupts chemo-responsive behaviour in an intertidal crustacean

Chemoreception is a key activity by which many aquatic animals perceive their environment, and therefore abiotic disruptions to this process could have serious impacts on the survival and fitness of individuals, and on species interactions. Hermit crabs are subject to cyclical reductions in the pH of the water in the intertidal rock pools that they inhabit. Such reductions may be further exacerbated by ongoing ocean acidification and/or leakage of carbon dioxide from geological storage sites and coastal upwelling events. Here we test the chemo-sensory responses of the hermit crab Pagurus bernhardus (Linnaeus) to a food odour under reduced pH conditions (pHNBS = 6.80). Acidifying the odour had no effect on its attractiveness indicating no permanent degradation of the cue; however, the pH of the sea water did affect the crabs' responses. Hermit crabs kept and tested in reduced pH sea water had lower antennular flicking rates (the ‘sniffing’ response in decapods); were less successful in locating the odour source, and showed an overall decline in locomotory activity compared to those in untreated sea water. Analysis of their haemolymph revealed a greater concentration of chloride ions ([Cl−]) in the reduced pH treatment group, suggesting iono-regulatory disruption; however, there was no correlation between [Cl−] and locomotory activity, suggesting a specific effect on chemoreception. This study shows that the chemo-responsiveness of a crustacean may be influenced by both naturally occurring pH fluctuations and future anthropogenically-induced changes in ocean pH.

Recruitment in a changing environment: the 2000s North Sea herring recruitment failure

Environmentally induced change appears to be impacting the recruitment of North Sea herring (Clupea harengus). Despite simultaneously having a large adult population, historically low exploitation, and Marine Stewardship Council accreditation (implying sustainability), there have been an unprecedented 6 sequential years of poor juvenile production (recruitment). Analysis suggests that the poor recruitment arises during the larval overwintering phase, with recent survival rates greatly reduced. Contemporary warming of the North Sea has caused significant changes in the plankton community, and a recently identified regime shift around 2000 shows close temporal agreement with the reduced larval survival. It is, therefore, possible that we are observing the first consequences of this planktonic change for higher trophic levels. There is no indication of a recovery in recruitment in the short term. Fishing mortality is currently outside the agreed management plan, and forecasts show a high risk of the stock moving outside safe biological limits soon, potentially precipitating another collapse of the stock. However, bringing the realized fishing mortality back in line with the management plan would likely alleviate the problem. This illustrates again that recruitment is influenced by more than just spawning-stock biomass, and that changes in other factors can be of equal, or even greater, importance. In such dynamically changing environments, recent management success does not necessarily guarantee future sustainability.

Macrofauna along the Sudanese Red Sea coast: potential effect of mangrove clearance on community and trophic structure

Mangroves along the Sudanese Red Sea coast are under constant anthropogenic pressure. To better understand the influence of mangrove clearance on the intertidal benthic community, we investigated the composition, biodiversity and standing stock of the macrofauna communities at high-, mid- and low-water levels in three contrasting habitats: a bare sand flat, a cleared mangrove and an intact mangrove. In addition, a community-wide metric approach based on taxon-specific carbon and nitrogen isotope values was used to compare the trophic structure between the three habitats. The habitats differed significantly in terms of macrofaunal standing stock, community composition and trophic structure. The high- and mid-water levels of the intact mangroves showed a distinct macrofaunal community characterized by elevated densities and biomass, largely governed by higher decapod and gastropod abundances. Diversity was similar for cleared and intact mangroves, but much lower for the bare sand flat. Community-wide metrics indicated highest trophic diversity and community niche breadth in the intact mangroves. Differences between the cleared and intact mangroves can be partly attributed to differences in sediment characteristics resulting from mangrove clearance. These results suggest a significant impact of mangrove clearance on the macrofaunal community and trophic structure. This study calls for further investigations and management actions to protect and restore these habitats, and ensure the survival of this ecologically valuable coastal ecosystem.