Seasonal and spatial variation in the diet of brown trout (Salmo trutta L.) in the subarctic River Laxá, north-east Iceland

This study examines seasonal and spatial patterns in the diet composition, stomach fullness and condition of a landlocked population of brown trout (2.5–61.3 cm) in the lake-fed River Laxá in NE-Iceland. The stomach contents consisted predominantly of benthic invertebrates, such as blackfly larvae Simulium vittatum (58%), chironomids (24%) and the freshwater snail Radix peregra (7%). The seasonal and spatial patterns of S. vittatum in the stomach contents were consistent with published studies on the life cycle and production of S. vittatum in the River Laxá. Close to the Lake Myvatn outlet (0–16 km), S. vittatum constituted 63% of the stomach content volume, and exhibited two peaks (i.e., generations) in its seasonal contribution to the stomach contents, whereas further downstream (16–35 km) S. vittatum constituted 45% of the volume and showed only clear evidence of one generation. Seasonal fluctuations in S. vittatum availability appear to affect the stomach fullness and the condition factor of the trout, especially close to the lake outlet where the fluctuations are pronounced.     

Seasonal variations in the physiological condition of the benthic amphipods Monoporeia affinis and Pontoporeia femorata in the Gulf of Riga (Baltic Sea)

Seasonal variations in the physiological condition of two benthic deposit-feeding amphipods were studied in two environmentally differing areas in the Gulf of Riga (Baltic Sea) with the purpose of estimating their role in benthic mineralisation. At the shallower (26 m) nearshore station, Monoporeia affinis exhibited low ammonia excretion rate (VNH ₄ ⁺ ) throughout the year. After intensive accumulation of lipids in spring, the lipid reserves of the individuals became rapidly depleted during autumn. Gravid females in winter-early spring showed exceedingly low lipid and triacylglycerol (TAG) levels compared to studies carried out in open-sea areas. Pontoporeia femorata studied at the deeper (43 m) offshore station exhibited a higher VNH ₄ ⁺ at all times with no significant seasonal variation. Lipid and TAG accumulated continuously until November, reaching higher levels than recorded previously for this species. Experiments showed that the temperature coefficient Q ₁₀ determined for VNH ₄ ⁺ is likely to depend on the recent feeding history of the amphipods. Mineralisation of nitrogen and carbon by the amphipod populations at the study sites was estimated to be 1.7-3.0 g C and 11.3-13.5 mmol NH ₄ ⁺ m^–2 yr^–1. Conclusively, both biotic and abiotic factors affect the condition of the amphipods, while the species also have genuine differences in their metabolic characteristics.     

Longitudinal patterns of phosphorus and phosphorus binding in sediment of a lowland lake–river system

In the Warnow River and its tributaries in North Germany, measurements were made to characterise the longitudinal patterns of nutrients in the riverbed and lake sediments. The sediment composition was analysed based on dry weight, organic matter, mean grain size and concentration of carbon, nitrogen, phosphorus, iron, aluminium and sulfur. Sediment phosphate was investigated in more detail by means of a sequential chemical extration. The phosphate was differently bound to the sediment particles in the upstream region than in the impounded section of the Warnow River and ist tributaries. Accumulation of fine sediment with high P-concentrations was recorded in the lake sediments and in the impounded section of the river. These impounded sections were the most important P-pool in the whole catchment area and played an important role in P-retention in the river system. Organic matter concentration, P-accumulation and P-binding in the sediment of the impounded section is corresponding with those of lake sediments. During the summer, anoxic P-release from the sediment in the impounded section was measured and calculated. The reductant-soluble fraction of the P-fractionation underestimated the release under anoxic condition. Adsorbed phosphorus and organic phosphate play an important role in P-release in the impounded part of the river.     

Atlantic paleobathymetry, paleoproductivity and paleocirculation in the late Albian: the benthic foraminiferal record

Spatial distribution patterns of benthic foraminifers in upper Albian sediments from 25 DSDP/ODP sites and 31 onshore sections of the North and South Atlantic Ocean are used to generate paleobathymetric reconstructions and to identify areas of high primary production such as coastal and equatorial upwelling zones. New paleobathymetric estimates are provided for DSDP/ODP sites and onshore locations that are not situated on oceanic crust. Paleobathymetric reconstructions indicate shallow water exchange between the North and South Atlantic but show the existence of a deep-water connection between the western and eastern Tethys (>2500 m) through the Gibraltar Gateway. Strikingly, there is no evidence for a strong latitudinal gradient in deep-water benthic foraminiferal distribution during the late Albian: South Atlantic assemblages show close affinity to North Atlantic and Tethyan assemblages, exhibiting only a minor degree of provincialism. Biogeographic patterns reveal a distinct asymmetry in late Albian paleoproductivity for the North Atlantic. As for the present day, the eastern margins of the Atlantic were generally more productive than the western margins, and a belt of enhanced carbon flux export to the seafloor can be traced around the north African coast, which probably corresponded to a zone of vigorous coastal upwelling. By contrast, assemblage composition in the South Atlantic generally reflects mesotrophic to oligotrophic conditions. Benthic foraminiferal distribution patterns, thus, provide robust proxy data to test predictions from paleocirculation and paleobathymetric models for the mid-Cretaceous Atlantic Ocean and adjacent margins.     

Carbohydrate synthesis by two Navicula strains isolated from benthic and pelagic mucilages in the Tyrrhenian Sea (Tuscan Archipelago)

Two exopolysaccharide (EPS)-producing strains of the diatom Navicula, were isolated from benthic and pelagic mucilaginous aggregates sampled in the Tyrrhenian Sea and cultured under laboratory conditions. The amount of carbohydrate formed over the growth period and on a per cell basis was quite similar. However, the benthic strain showed a preferential synthesis of the bound (i.e., cellular and capsular) carbohydrate fraction, whereas the pelagic strain preferentially synthesised soluble, polymeric carbohydrates. The polysaccharides released into the medium by the two strains showed the same qualitative monosaccharidic composition, being constituted by two acidic and six neutral sugars. It is suggested that the difference between the benthic and the pelagic strain in the synthesis of bound or soluble carbohydrates may be related to the different role of these compounds in the particular habitats of the strains.     

A manipulative experiment to evaluate predicted changes in intertidal, macro-faunal assemblages after contamination by heavy metals

Concentrations of heavy metals (zinc, copper and lead) were manipulated experimentally to test the hypotheses about effects on intertidal, soft-sediment assemblages of animals in two sand-flats in Port Hacking, Australia. Hypotheses about changes in the structure and composition of whole assemblages and changes in mean abundance and variability of individual taxa were tested. Specific hypotheses were derived a priori from repeated observations of assemblages in urban and non-urban areas of Port Hacking.After manipulation, concentrations of metals were similar to those in sediments near urban areas. Nevertheless, responses of assemblages to the increased concentrations of metals were weak. Polychaetes, spionids and amphipods responded to experimental treatments. Changes were, however, not consistent among times and places and generally not in agreement with what had been predicted. Significant spatial and temporal variability were detected for all variables investigated. Increased concentrations of metals did not affect variability or overall structure of assemblages. Thus, there was little evidence that increased concentrations of metals caused benthic assemblages in pristine areas to become more similar to those in areas contaminated by human activities.Several potential explanations for the discrepancy between previously observed correlative patterns and the results presented here are discussed. These include a critical assessment of different aspects of the experimental study, such as lack of statistical power, insufficient basis for prediction, artefacts due to experimental procedures and issues to do with the difference between evidence based on correlation and manipulation. Explicit comparisons showed that there were significant effects of physical disturbance due to repeated sampling and that assemblages of animals at the start of the experiment were different from those previously observed in uncontaminated areas. These observations were surprising and made interpretation more difficult. Nevertheless, it is possible that the metals investigated really contribute only marginally to previously observed differences between urban and non-urban areas. Repeated comparisons between observations of patterns and manipulative experiments like these can only improve the basis for prediction and the power of current mechanistic models.     

Kelp rafts in the Southern Ocean

Kelp rafts were surveyed during three summer traverses between Hobart and Macquarie Island (subAntarctic) to determine the potential for dispersal of kelp‐associated macroinvertebrates across the latitudes of 46–53°S. Rafts of Durvillaea antarctica dominated the sightings (94% of rafts) and extrapolations to the full Southern Ocean between these latitudes indicate a figure of over 70 million rafts afloat at any one time, 20 million of which support a holdfast, the habitat supporting the highest faunal diversity in attached kelp plants. In contrast, few, small rafts of Macrocystis pyrifera were observed. The potential for dispersal of fauna is presumed to be related to the species of kelp with which they are associated. Empirical studies of survival of animals while drifting at sea, and also on making land‐fall, are required to allow fuller interpretation of the significance of these findings.     

Benthic fauna in tropical tidal flats of Hinchinbrook Channel, NE Australia: diversity, abundance and their spatial and temporal variation

A survey of the benthic infauna (macro- and mesofauna) in tidalflats of Hinchinbrook Channel, north-east Australia, was carried out toassess the species diversity and individual abundances as well as theirtemporalvariations. Two sites were surveyed at five occassions from November 1988 toOctober 1991. In addition, spatial distributions over the intertidal gradientwere investigated once in May 1990. During the entire survey, over 200 specieswere recorded, with 53 species found on average at each sampling occassion.Species densities ranged from 4.0 to 8.2 species 177cm^–2 for macrofauna and from 4.1 to 11.8 species10 cm^–2 for mesofauna, but varied little betweensites. The average infaunal diversity (H') was 2.53. Individual abundancesreached median values of 14.0 individuals 177cm^–2for macrofauna and 14.5 individuals 10 cm^–2 formesofauna. Changes in total abundances were not related to season andinconsistent at the two study sites, with significantly higher abundancesoccurring either at site A or B on single sampling dates. Transects sampled inMay 1990 showed varying species compositions and abundances in the upper, midand lower intertidal, but no defined benthic communities were revealed bymultivariate analyses. Temporal variations in the benthic assemblages of thetwotidal flats in Hinchinbrook Channel were higher than variations between sites,with a separation of assemblages before and after 1989. Problems to assess andinterpret temporal changes in species rich tropical benthic communities arediscussed.     

Benthic nematodes in acidified lakes: case of a neglected grazer

Abundance and composition of nematode fauna were examined in the benthic microbial mats and upper sediment layer of the littoral of acidified (pH 4.6 to 5.7) lakes. Nematodes constituted from 58% to 90% of all the invertebrates present (excluding protozoans and rotifers). In the examined material, the majority of nematodes was represented by 3 taxa of which Ironus was found specifically associated with the mats. It was concluded that the persistence of benthic mats may be linked to the metabolic activity of the associated nematode fauna.     

Light-dependence of the metabolic balance of a highly productive Philippine seagrass community

Seagrass meadows are important primary producers in SE-Asia coastal areas that are increasingly threatened by human activities resulting in a deterioration of the underwater light environment. The resilience of seagrass meadows to decreasing light availability should be approached in an integrative manner, because they shelter complex communities of primary and secondary producers. The aim of this study was to measure the in situ metabolism of a seagrass community under different levels of light availability following changes in the water column dissolved oxygen (DO) and dissolved inorganic carbon (DIC), the sediment redox potential and seagrass production. Net community production (NCP) and respiration were measured along two diel cycles to produce a balance of NCP under different light treatments. On a daily basis, at full irradiance, the community metabolism presented a net production which was close to zero, with values of −7.75 to 16.6 mmol O₂ m⁻² day⁻¹ for DO, and −56.8 to 22.7 mmol C m⁻² day⁻¹ for DIC in the first and second incubation runs, respectively. Compensation irradiance for the NCP was thus found to be close to 80% of the present light availability. Shading resulted in a general decrease in the sediment redox potential, while the initial redox potential had not recovered 6 days after exposure to full sunlight. This community appears to be in a fragile equilibrium with the environment, and any minor decrease in the water transparency would lead to a shift from an autotrophic to a heterotrophic system.