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. In the period 2005-2008 it has been studied within the MarBEF project DEEPSETS.

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 megafauna 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). Their species composition 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.

Macrofauna

Macrofaunal polychaetes exhibited a more muted response to changes at the Porcupine Abyssal Plain (PAP). Although their abundance increased significantly before and during the Amperima event, it was not on the same scale as that observed in the megafauna. Moreover, only certain taxa and trophic groups responded.

Only six of the twelve most abundant species showed a significant response (increased abundance ) during the Amperima event. The fact that only some polychaete species responded may be related to the efficient foraging by megafaunal deposit feeders, leaving less available for smaller organisms.

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.

The deep Arctic

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. For the larger organisms, a towed camera system revealed a significant decrease in megafauna densities at 2.500m water depth.