Seasonal growth and senescence in continental shelf ecosystems: a test of the SEEP hypothesis

The hypothesis that continental shelf ecosystems export a major fraction of the carbon produced by the phytoplankton during the spring bloom was tested during the Shelf Edge Exchange Processes (SEEP) experiment off the north-east coast of the United States in 1984. The total benthic standing stocks in terms of organic carbon (macrofauna, meiofauna and bacteria) have been estimated in the SEEP area. Their preponderance on the continental shelf was partial evidence that little organic matter escapes to the upper continental slope. Measurements of the metabolism of the biota allowed calculation of turnover times of organic detritus and the total biota. The turnover time of detritus increased as grain size decreased, suggesting that fine-grained deposits contain mostly refractory, non-reactive compounds, especially on the deep slope. Turnover times of the total biota were about the same in the coarse- as in the fine-grained shelf deposits, but a far larger fraction of the turnover was attributed to the bacteria in the fine sediments. On average, about 25 per cent of the primary production appeared to be utilized by the aerobic benthos on the continental shelf in the SEEP area. The role of anaerobes at depth in the sediments remains uncertain. This study, along with a review of other recent work, leads to the conclusion that only a small fraction of continental shelf phytodetritus is exported across a distinct shelf-slope hydrographic frontal system. What is not consumed in the spring is utilized on the shelf during the ensuing stratified season. More open ended ecosystems may export production more readily.     

Review of the potential use of brown algal ecotoxicological assays in monitoring effluent discharge and pollution in southern Australia

The temperate near-shore reefs of southern Australasia possess diverse assemblages of brown algal macrophytes which underpin major grazing and detrital food chains in medium to high energy coastal habitats. These ‘kelp' communities are at risk from effluent discharge with documented decline of macrophyte stands in areas subject to outfall. Recent studies utilising reproductive endpoints of several Australian species in ecotoxicological assays have highlighted their sensitivity to pollution. Results indicate an acceptable degree of reproducibility for assays with variation in response between taxa presumably related to differing habitat requirements and potential for stress resulting from toxicant/pollution exposure. Species studied are drawn from the orders Laminariales and Fucales, with the latter in particular offering substantial scope for further investigation. The greatest diversity of fucoid algae is found in south-eastern Australia and closely related taxa occur in similar latitudes in both the southern and northern hemispheres. The broad distribution of taxa from both taxonomic groups enables ready comparison of assays from disparate locations and offers the potential for simple, routine toxicological testing and habitat monitoring across broad geographic ranges.     

Algal interactions on shallow subtidal reefs in northern New Zealand: A review

In north‐eastern New Zealand, nearshore subtidal reefs are dominated by large brown algae of the orders Fucales and Laminariales. Species of the genera Carpophyllum, Sargasswn, and Landsburgia are the most conspicuous fucaleans, whereas Ecklonia radiata is the only common laminarian. Three categories of events affect the composition of stands of these algae: first, the competitive effects of adult canopies on recruitment to the substratum below; second, the seasonality of spore release and availability of free space; and third, the demographic characteristics of species and their influence on recruitment. Adult canopies suppress recruitment of all species. Both repro‐duction and recruitment are seasonal in occurrence and most recruits appear within a few metres of adults of their own species. A comparison of representative Fucales and Laminariales shows important differences in how these taxa colonise substrata. Propagules of fucaleans are larger at settlement and fewer in number than those of laminarians. Compared to laminarians, however, higher numbers of fucalean adults are generally required to form a closed canopy. The number of recruits and the survival of each life history stage are affected by the population structure of algal stands. The importance of the demographic characteristics of individual species in assessing these processes is highlighted.     

The rise of Laminaria ochroleuca in the Western English Channel (UK) and comparisons with its competitor and assemblage dominant Laminaria hyperborea

The distribution of species is shifting in response to recent climate change. Changes in the abundance and distributions of habitat‐forming species can have knock‐on effects on community structure, biodiversity patterns and ecological processes. We empirically examined temporal changes in the abundance of the warm‐water kelp Laminaria ochroleuca at its poleward range edge in the Western English Channel. Resurveys of historical sites indicated that the abundance of L. ochroleuca has increased significantly in recent decades. Moreover, examination of historical records suggested that L. ochroleuca has extended its distribution from sheltered coasts on to moderately wave‐exposed open coasts, where it now co‐exists and competes with the assemblage dominant Laminaria hyperborea. Proliferation of L. ochroleuca at its poleward range edge corresponds with a period of rapid warming in the Western English Channel. Preliminary comparisons between L. ochroleuca and L. hyperborea highlighted some subtle but ecologically significant differences in structure and function. In summer, the average biomass of epiphytic stipe assemblages on L. hyperborea was 86 times greater than on L. ochroleuca whereas, on average, L. ochroleuca had a greater stipe length and its blade supported 18 times as many gastropod grazers (Gibbula cineraria). Differences in summer growth rates were also recorded, with L. ochroleuca being more productive than L. hyperborea throughout July. Comprehensive seasonally replicated comparisons are needed to examine the wider implications of proliferation of L. ochroleuca at its poleward range edge, but our study suggests that local biodiversity patterns and ecological processes (e.g. timing of productivity and trophic pathways) on shallow subtidal reefs may be altered by shifts in the relative abundances of habitat‐forming kelp species.