Temporal variability in living deep-sea benthic foraminifera: a review

Abstract

The deep ocean environment is disturbed by various processes, many of which involve episodic inputs of organic matter. Some inputs (e.g., phytodetritus at mid-high latitudes in the North Atlantic and Northeast Pacific) are seasonally pulsed, others (e.g., falls of whale carcasses) are irregular and unpredictable, but together, they evoke a variety of responses from the benthic biota. In the case of deep-sea foraminifera, only those responses arising from seasonal food pulses have been fairly well-documented. The population dynamics of deep-sea benthic foraminifera (total live populations and individual species) appear to be controlled largely by two inversely-related parameters, the flux of organic matter to the seafloor and concentrations of oxygen in the sediment porewater. Organic matter (food) inputs are most intense along bathyal continental margins, and their oxidation often leads to the depletion of oxygen in surface sediments. Under these conditions, foraminiferal faunas are dominated by low-oxygen tolerant, infaunal species, the abundance of which fluctuate in response to seasonally varying amounts of food and oxygen. At some sites (e.g., Sagami Bay, off Japan), species migrate up and down in the sediments, tracking critical oxygen concentrations. Where oxygen concentrations are consistently low (less than about 0.5 ml l −1), as in parts of the California Borderland, foraminifera may undergo population increases solely in response to food pulses. In the abyssal North Atlantic, and in some continental margin areas of this ocean, organic matter inputs are weaker and do not lead to oxygen depletion within surface sediments. These systems are food limited and seasonal population fluctuations reflect the availability of food (phytodetritus) rather than oxygen. Here, the species which respond to phytodetritus are mainly epifaunal or shallow infaunal opportunists which represent a small proportion of highly diverse communities (2 or 3 out of >120 species per core of 25.5 cm 2 surface area). Seasonal phytodetrital pulses to the deep-seafloor, and hence, foraminiferal population dynamics, are not entirely predictable. Being dependent on climatic and upper-ocean processes, they vary in intensity from year to year and occasionally (e.g., at the Porcupine Abyssal Plain (PAP) in 1997) fail to materialise. Foraminiferal responses to irregular (non-seasonal) organic matter inputs are poorly-known. However, there is some evidence that whale falls, turbidite deposits, hydrothermal vents and seeps are exploited by species typical of organically-enriched, low-oxygen environments rather than by a specialised fauna. Fossil foraminiferal assemblages from bathyal and abyssal environments may provide evidence for an increase or decrease in the seasonality of surface production as well as for longer-term changes in palaeoproductivity. However, the accurate interpretation of this record depends on filling the many gaps which remain in our understanding of relations between benthic foraminiferal ecology and seasonal phenomena in the deep ocean.