Effects of climate change on the North Atlantic benthos

The Amperima event

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. Recently it has been studied within the MarBEF DEEPSETS project.

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 the amount and quality of particulate organic carbon (POC) that is exported from the surface layer to the sea floor. These changes in food quantity and quality probably explain the ‘boom-bust’ cycles (rapid abundance increases followed by declines) observed in the holothurians Amperima rosea and Ellipinion molle, during the period from 1996 to 2005. The rise to dominance of A. rosea during 1996 has been called the ‘Amperima event.’

Two larger holothurian species, Psychropotes longicauda and Pseudostichopus aemaulatus, increased more slowly, while a third, Oneirophanta mutabilis, underwent a significant decrease over the entire observation period. 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, there was little phytodetritus on the seafloor between 1997 and 1999.

Impact on foraminifera

The densities of foraminifera were significantly higher in the post-Amperima event period (1996- 2002) compared to the pre-Amperima event period (1989-1994). The species composition of the foraminifera also changed over this period. 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 was observed. This may have reflected the qualitative change in the phytodetrital food, the repackaging of food by megafauna, the increased megafaunal disturbance, or a combination of these factors.

Thus, the PAP time-series suggests that the decadal-scale changes among the shallow-infaunal foraminifera, more or less coincided with changes in the megafauna, as well as with shorter term events related to seasonally-pulsed phytodetrital inputs.

Schematic diagram illustrating forcing factors that influence temporal processes in ’normal’ sedimented parts of the deep-sea and in chemosynthetic systems. In the first case, temporal changes are forced ultimately by climatic oscillations. In the second case, they are forced by geological processes that affect fluid flow.

Meiofauna

Densities of animal meiofauna increased significantly between 1989 and 1999, mainly because of increases in the dominant taxon, the nematodes, and to a lesser extent the polychaetes. Ostracods showed a significant decrease while most other taxa, including the copepods, did not exhibit significant temporal changes in abundance.

The proportion of nematodes and copepods (but not polychaetes) which inhabited the 0-1cm layer of the sediment also increased in time. The vertical distribution of nematodes also showed seasonal variations during the intensively sampled 1996-97 period.

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.

Shifts in the benthic biota of the deep-sea communities at the Porcupine Abyssal Plain have been recorded over decadal as well as shorter (seasonal) time-scales and attributed to the North Atlantic Oscillation.

Vastly increased populations of these small surface feeding organisms may, have affected foraminiferal and meiofaunal populations by depleting food resources and disturbing the sediments. A similar relationship between climate, sea-surface processes and deep-sea benthos appears to exist in the North East Pacific Ocean.