Effects of climate change on the North Atlantic benthos

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In the North Atlantic, temporal changes in deep-sea communities at the Porcupine Abyssal Plain (PAP), at 4,850m water depth, have been studied since 1989, most recently within the DEEPSETS project (Fig. 3). Shifts in different elements of the benthic biota of the deep-sea communities at the Porcupine Abyssal Plain over decadal as well as shorter (seasonal) time-scales have been recorded and attributed to the North Atlantic Oscillation (a climatic phenomenon). While intra-annual changes reflect seasonal productivity cycles, the decadal-scale changes at the PAP are believed to be linked to the North Atlantic Oscillation, a climatic phenomenon that affects winds, precipitation and storm intensity and frequency. These oscillations lead to changes in upper ocean biology and the export of particulate organic carbon (POC) from the euphotic zone (i.e., the export flux) and to the sea floor, as well as in the quality (biochemistry) of the material that reaches the sea floor. These changes in food quantity and quality (for example, the content of pigments necessary for reproduction) probably underlie the ‘boombust’ cycles observed in the holothurians Amperima rosea and Ellipinion molle. Vastly increased populations of these small surfacefeeding organisms may, in turn, have affected foraminiferal and meiofaunal populations through depletion of food resources and sediment disturbance. A similar relationship between climate, sea-surface processes and deep-sea benthos appears to exist in the NE Pacific Ocean. The most obvious changes at the PAP were seen among the megafauna (animals visible in seabottom photographs and trawls), notably the holothurians Amperima rosea and Ellipinion molle. These relatively small species both exhibited ‘boom-bust’ cycles - rapid abundance increases followed by declines – during the period from 1996 to 2005. The rise to dominance of A. rosea during 1996 has been termed the ‘Amperima event.’ Two larger holothurian species, Psychropotes longicauda and Pseudostichopus aemulatus, exhibited more modest increases while a third, Oneirophanta mutabilis, underwent a significant decrease over the entire time-series. Increases in holothurian densities led to a dramatic increase in the extent to which surface sediments, and particularly deposits of phytodetritus (organic detritus derived from surface primary production), were reworked. Probably as a result of these activities, there was little sign of phytodetritus on the seafloor between 1997 and 1999. Among smaller organisms, densities of foraminifera were significantly higher in 1996- 2002 (post-Amperima event) compared to 1989-1994 (pre-Amperima event). The species-level composition of the assemblages changed over this period, reflecting fluctuations in the densities of higher taxa and species. In 1996, following a phytodetritus pulse, the miliolid Quinquiloculina sp. migrated to the sediment surface, grew and reproduced before migrating back into deeper layers as the phytodetrital food became exhausted. A substantial increase in the abundance of trochamminaceans, notably one small, undescribed species, may have reflected qualitative change in the phytodetrital food, repackaging of food by megafauna, increased megafaunal disturbance of the surficial sediment, or a combination of these factors. Thus, the PAP time-series suggests that decadal-scale changes have occurred among shallow-infaunal foraminifera at this site, more or less coincident with changes in the megafauna, as well as indications of shorterterm events related to seasonally-pulsed phytodetrital inputs. Densities of metazoan meiofauna increased significantly between 1989 and 1999, driven mainly by the dominant taxon, the nematodes, and to a lesser extent the polychaetes. Ostracods showed a significant decrease while most other taxa, including the second-ranked group, the copepods (harpacticoids and nauplii), did not exhibit significant temporal changes in abundance. MDS ordination of higher taxon composition showed a significant shift from the earlier (pre-Amperima, 1989- 1994) to the later (1996-1999, post-Amperima) periods. There were also significant increases over time in the proportion of total meiofauna, nematodes and copepods (but not polychaetes) inhabiting the 0-1cm layer. In addition, seasonal changes in the vertical distribution patterns of total meiofauna and nematodes within the sediment were apparent during the intensively sampled period, 1996-97. Macrofaunal polychaetes exhibited a more muted response to changes at the Porcupine Abyssal Plain. Although the abundance of the whole assemblage increased significantly before and during the Amperima event, the increase was not on the same scale as that observed in the megafauna, and only certain taxa and trophic groups responded. The same dominant species occurred throughout the study period, with the exception of the Paraonidae, where the dominant species declined prior to the Amperima event and was replaced by two other species. Only six of the 12 most abundant species showed a significant response (abundance increase) during the Amperima event. The fact that only some polychaete species responded may be related to efficient foraging by megafaunal deposit feeders that sequestered and repackaged organic matter, leaving less available for smaller organisms. Yet there did not appear to be an impact from physical disturbance caused by megafaunal feeding activities. For example, surface deposit feeders increased during the Amperima event at the same time as disturbance of the surficial sediment by holothurians and ophiuroids was also increasing. The polychaetes indicate that changes in the upper ocean which affect the ocean floor may operate in a complex way and that high taxonomic resolution is needed to establish how the fauna responds. Temporal changes in the deep sea are not confined to the deep Abyssal Plains; changes have also been recorded in the Arctic and the Mediterranean. In the Arctic, work by the Alfred-Wegener Institute in Bremerhaven demonstrated a small but important temperature increase between 2000 and 2008 at 2,500m depth in the Fram Strait between Svalbard and Greenland. Within DEEPSETS, a five-year (2000-2004) time-series study of nematodes at this site revealed shifts in nematode abundance and community composition, reflecting changes in food availability. Although depth-related changes were more prominent than shifts relating to sampling year, interannual variability in nematode community structure was clearly apparent, particularly at the 4,000m station. Parallel observations at several water depths indicated that most of the variation over the time-series was the result of real temporal changes, driven by shifts in food availability as measured by sediment-bound phaeopigment and chlorophyll a concentrations. For the larger organisms, a towed camera system revealed a significant decrease in megafauna densities at 2,500m water depth.