SYMBIONT-BEARING FORAMINIFERS AS BIOINDICATORS FOR CORAL REEFS: USES AND LIMITATIONS

Foraminifers are important components of coral-reef biotas. Reef-dwelling taxa include algal symbiont-bearing species, which tend to be large in size and most abundant and diverse in oligotrophic waters with substantial oceanic influence. Smaller heterotrophic taxa tend to be most diverse and abundant where terrestrial input or upwelling increase nutrient flux and therefore food supply. Several parameters based on foraminiferal assemblages can be useful indicators of water quality and the potential for reef hypercalcification. These parameters include abundance and diversity of larger foraminiferal shells in reef sediments in comparison both to other foraminiferal shells and to other sediment constituents. However, local, regional and global environmental changes associated with human activities are impacting symbiont-bearing foraminifers somewhat differently than reef building corals. Stratospheric ozone depletion has increased relative proportions of high energy, shorter wavelengths of light reaching the shallow sea floor at the same time global warming is increasing sea-surface temperatures; both factors cause photo-oxidative stress that can induce bleaching and increase susceptibility to disease. In the western Atlantic, coral cover paradoxically has declined more slowly on inshore patch reefs where water transparency is lower and temperature more variable, than on reef margins at similar depths. Symbiont-bearing foraminifers continue to be most abundant in clear, offshore reef environments, even though they have suffered population crashes associated with acute bleaching events. Longevities appear to be critical to explaining recovery differences of larger foraminifers and stony corals to bleaching and disease. Larger foraminiferal populations can recover in a year or two following a high mortality event; corals may require a decade or more. Moreover, the acute sensitivity of certain foraminifers to photic stress, combined with slower decline of corals in less transparent waters, argue for a greater role for photic stress than is generally assumed by coral researchers. Thus, recovery of the stratospheric ozone layer, mandated by international treaty, provides a glimmer of hope in the generally poor prognosis for the future of coral reefs.