ALBERT

Description: Effects of elevated temperature on the formation and subsequent degradation of diatom aggregates were studied in a laboratory experiment with a natural plankton community from the Kiel Fjord (Baltic Sea). Aggregates were derived from diatom blooms that developed in indoor mesocosms at 2.5 and 8.5 degrees C, corresponding to the 1993 to 2002 mean winter in situ temperature of the Western Baltic Sea and the projected sea surface temperature during winter in 2100, respectively. Formation and degradation of diatom aggregates at these 2 temperatures in the dark were promoted with roller tanks over a period of 11 d. Comparison of the 2 temperature settings revealed an enhanced aggregation potential of diatom cells at elevated temperature, which was likely induced by an increased concentration of transparent exopolymer particles (TEP). The enhanced aggregation potential led to a significantly higher proportion of particulate organic matter in aggregates at 8.5 degrees C. Moreover, the elevated temperature favoured the growth of bacteria, bacterial biomass production, and the activities of sugar- and protein-degrading extracellular enzymes in aggregates. Stimulating effects of rising temperature on growth and metabolism of the bacterial community resulted in an earlier onset of aggregate degradation and silica dissolution. Remineralization of carbon in aggregates at elevated temperature was partially compensated by the formation of carbon-rich TEP during dark incubation. Hence, our results suggest that increasing temperature will affect both formation and degradation of diatom aggregates. We conclude that the vertical export of organic matter through aggregates may change in the future, depending on the magnitude and vertical depth penetration of warming in the ocean.

Description: Models of multiple potentially limiting nutrients currently employ either multiplicative or threshold formulations, neither of which has a sound mechanistic explanation. Despite experimental evidence that lack of P severely constrains N assimilation, this mechanism has not been considered for constructing models of multi-nutrient limitation. We construct a phytoplankton optimal growth model linking C, chlorophyll (Chl), N, and P through a limitation chain in which P limits N assimilation, N limits photosynthesis and photosynthesis limits growth. The resulting formulation possesses characteristics of both multiplicative and threshold approaches and provides a mechanistic foundation for modelling multi-nutrient and light limitation of phytoplankton growth. The model compares well with experimental observations for a variety of unicellular phytoplankton species. It is suggested that the widely held view that N and P limitation act independently of each other is based on an invalid interpretation of experimental observations and that the transition from N to P limitation occurs over a wide range of colimitation rather than a sharply-defined transition point. If the species considered in this study are representative for marine phytoplankton, our model results indicate that most phytoplankton are colimited by N and P when inorganic N and P are simultaneously exhausted in the surface ocean. The model suggests that the close match between marine inorganic (Redfield) and phytoplankton N:P ratios results from optimal nutrient utilisation but does not indicate optimality of Redfield N:P.

Description: Generalist and opportunistic marine predators use flexible foraging behaviour to exploit prey bases that change in diversity and spatial and temporal distributions, Behavioural flexibility is constrained by characteristics Such as individual cognitive and physical capabilities, age, reproductive condition and central place foraging. To assess flexibility in the foraging tactics of a marine bird, we investigated the diets and foraging behaviour of the largest seabird predator in the North Atlantic Ocean. Northern gannets Sula bassana exploit abroad spectrum of pelagic prey that range in mass by more than 2 orders of magnitude, We investigated their foraging activity at their largest. offshore colony in the western Atlantic Ocean during 1998 to 2002, when they preyed primarily on shoals of spawning and post-spawning capelin Mallotus villosus, a small forage fish (similar to 15 g), and also on a much larger pelagic fish, post-smolt Atlantic salmon Salmo salar (similar to 200 g). Inter-annual dietary variation is associated with gannet and prey fish distributions. Landings of capelin at the colony by gannets were correlated with returns of larger foraging flocks from inshore, whereas landings of Atlantic salmon were associated with smaller flocks returning from offshore. Maximum foraging trip distances ranged from 20 to 200 km and averaged 57 +/- 12 (SE) km, consistent with distances to inshore capelin aggregations. When capelin abundance was low (in 2002), more gannets foraged offshore, preyed on large pelagic fishes (mostly Atlantic salmon) and exhibited the greatest dietary diversity. Though the Outbound portions of foraging trips were more sinuous than inbound routes, individual gannets exhibited general fidelity to foraging sites. These large avian predators used flexible foraging tactics to adjust to changing prey conditions and generate longer-term strategies to Lake advantage of diverse trophic interactions over a range of ocean ecosystems.

Description: Individual migratory schedules and wintering areas of northern gannets Morus bassanus were studied over 2 consecutive winters by deploying geolocation data loggers on breeding adults from the Bass Rock, UK. Northern gannets attended the breeding colony on Bass Rock until between 24 September and 16 October (median: 5 October). Afterwards, individual birds engaged in different migratory behaviour. Of the 22 birds tracked until at least December, 18% wintered in the North Sea and the English Channel, 27% in the Bay of Biscay and the Celtic Sea, 9% in the Mediterranean Sea and 45% off West Africa. Individual winter home ranges as measured by the 75% kernel density contours varied between 8 100 and 308 500 km(2) (mean = 134 000 km(2)). Several northern gannets migrated northwards from Bass Rock after leaving the colony for a stay of a few days to a few weeks, independent of whether they migrated to Africa or other southern areas later. Birds wintering off West Africa migrated to their wintering areas mostly within 3 to 5 wk, usually starting between early and late October. Most of these birds stayed off West Africa for a period of about 3 mo, where they remained in a relatively restricted area. Return migration was initiated between the end of January and mid-February, and took about as long as autumn migration. We conclude that individual gannets display very variable migratory behaviours, with discrete winter home ranges, and we infer that the migration habits of gannets may be changing in response to human impacts on marine ecosystems.

Description: We examined the influence of both season and hydrographic and meteorological factors on seabird abundance in the southern North Sea. Seabirds were counted from ships in a study area of 27.8 x 32.8 km on 407 d from 1990 to 2007. Two hydrographic and 5 meteorological parameters were taken from archived data. The relationships between bird abundance and abiotic parameters were investigated by generalised additive models for 3 distinct seasons. The species in the study area exhibited different seasonal patterns. While some species were present year-round, others occurred only at certain periods. Despite these substantial changes in abundances, the nature of the interactions between bird abundances and abiotic parameters did not vary much between seasons. All 5 meteorological and 2 hydrographic parameters significantly influenced the abundance of seabird species, though to a different degree. The single factors that most often had a significant influence in the single models were wind field, sea surface temperature anomaly, sea surface salinity anomaly and air pressure change. The quantitative composition of the seabird community differed significantly between onshore wind and offshore wind conditions. It is assumed that hydrographic parameters are relevant for the birds by determining their foraging habitats and that atmospheric parameters influence flight conditions during foraging and migration.

Description: Ecological stoichiometry can be a powerful tool to understand food web consequences of altered biogeochemical cycles as well as consequences of biodiversity loss on biogeochemistry and has proved to be a suitable framework to predict effects of consumers on the nutrient content of their prey. However, predictions from ecological stoichiometry have mainly been tested using single consumer species, whereas in most natural ecosystems several consumer species coexist. We conducted 2 outdoor mesocosm experiments with marine rock pool communities to test whether species richness and species combination of benthic invertebrates affected the nutrient content of periphyton. We independently manipulated 12 different consumer combinations ranging from 0 to 6 (2004) or 0 to 4 (2005) grazer species and measured the biomass and nutrient content of the algae. Grazers included 3 gastropods and 3 crustaceans. In 2005, we additionally analyzed animal nutrient content and N excretion rate. Algal biomass and C:N ratios decreased in the presence of grazers in both years, indicating that the remaining algae had higher internal N content. Also, both biomass (2004 and 2005) and C:N ratios (only 2004) decreased even further when grazer richness increased. In 2004, significant net diversity effects of grazer richness on periphyton C:N ratios indicated that periphyton N content under multispecies grazing could not be predicted from the effect of single species. In 2005, significant net diversity effects on C:N ratios were rare, but periphyton C:N ratios consistently decreased with increasing grazer excretion rate, indicating that higher nitrogen regeneration by grazers led to higher N incorporation by algae. The effects of species richness were mainly affected by the presence of one efficient grazer, the gastropod Littorina littorea. Our experiments indicate that non-additive intraguild interactions may qualitatively alter the stoichiometric effects of multispecies consumer assemblages.

Description: The relationship between physical properties of the water column and spatial patchiness of phytoplankton spring bloom development on the Greenland shelf edge and in the Irminger Sea was investigated using data collected during a spring cruise (April and May 2002). The observations confirm a strong relationship between the onset and stage of bloom development and the stratification induced by freshwater input to the surface layer in the shelf region. Interestingly, at the shelf, in the region influenced by melting of the seasonal ice-cover, the vertical distribution of chlorophyll a showed a subsurface maximum at ca. 25 m depth at several stations. Since nutrients were not exhausted at these stations, such a pattern does not conform to the general picture of a spring bloom. In contrast, in the open ocean part of the Irminger Sea pre-bloom conditions and a retarded development of the phytoplankton population were observed with low, more uniform distribution of chlorophyll a. The nitrate drawdown was estimated at between 16.5 and 270 µm m–2 (mean 108.6 ± 82.2 µm m–2) and the new primary production was estimated to be between 1.3 and 21.4 g C m–2 (8.6 ± 6.5 g C m–2), corresponding to 0.42 g C m–2 d–1. The phytoplankton community in the melting ice zone consisted of Phaeocystis sp., small flagellates (〈 4 µm) and picoplankton, while diatoms were less abundant. Phaeocystis sp. contributed up to 15 g C m–2 to the carbon biomass (70% of total carbon measured), whereas the contribution of diatoms and flagellates to carbon biomass was relatively low, with up to 1.2 g C m–2 (5.7%) and up to 2.5 g C m–2 (11.7%), respectively. On the shelf the bloom starts at the very beginning of stabilisation (elevated N2 values) which results solely from the release of meltwater. The locally restricted water stability leads to a patchy phytoplankton distribution in the Irminger Sea.

Description: The effect of ambient solar ultraviolet radiation (UVR) on a shallow-water (4 cm) tropical fouling community was assessed during the succession of macrobenthic species on artificial substrates at the Wong Shek fish farm, Hong Kong. The early successional communities developing under 3 radiation treatments (PAR + UV-A + UV-B = 280 to 700 nm; PAR + UV-A = 320 to 700 nm, and PAR = 400 to 700 nm) were monitored for 14 wk. A total of 8 species of algae and 8 species of invertebrates colonised the experimental tiles. During the first 8 wk of the experiments, there were no differences among treatments in diversity, percentage of cover of species and the biomass of the colonisers. During the following 6 wk, the communities exposed to UVR had lower species richness than the communities exposed to only PAR had. The species diversity (after 79 and 98 d) of the 3 treatments varied, but the total percentage of species cover and the entire community biomass were not significantly different across the experiment. Juveniles of the polychaete Hydroides elegans and the barnacle Balanus amphitrite, juveniles of the clams Perna viridis and Modiolus comptus, and the algae Enteromorpha sp., Ectocarpus sp. and Cladophora sp. were responsible for the dissimilarity between communities developed under different UVR treatments. The algae constituted a higher percentage of the cover under the full sunlight spectrum, whereas the polychaete, the barnacle and the clams were dominant in the no-UVR treatment. Our outdoor experiment revealed that UVR inhibited the settlement and decreased post-settlement survival of H. elegans. We concluded that UVR affects the composition of early successional, shallow water biofouling communities in tropical waters as well as the settlement and mortality of single species.

Description: It is widely assumed that the production of secondary metabolites against grazing and fouling is costly for seaweeds in terms of metabolic energy and should therefore be reduced under conditions of resource limitation. Here we tested the hypothesis that anti-herbivore defenses and bioactivity against mussels in 4 brown seaweeds from northern-central Chile will be reduced when light is limited. In a 2 wk experiment, seaweeds were kept under different low-light conditions (~76 to 99% reduction of ambient sunlight) and grazing situations. Subsequently, we tested their anti-herbivore defense against a common amphipod grazer in feeding assays with living algal tissue and reconstituted food pellets. A standard test employing the production of byssus threads by mussels was furthermore used as an indicator for deterrents in crude algal extracts. All investigated seaweeds showed decreased growth under the stepwise light reduction. Lessonia nigrescens exhibited reduced defense ability under severe low-light conditions when living tissue was offered to the amphipod, probably caused by changes in the tissue structure or in nutritional traits. In Dictyota kunthii, L. trabeculata and Macrocystis integrifolia this effect was absent. None of the investigated seaweeds showed a clear effect of light reduction on chemically mediated defenses against the mesograzer and there was no effect of light limitation on the bioactivity against mussels. Thus, against general assumptions, chemical defense in the investigated seaweeds does not appear to be reduced under severe resource limitation. Results suggest that seaweeds may use different strategies of energy allocation to cope with low-light conditions.