CURRENT DAY THREATS TO CORAL REEFS: NATURE OR NURTURE? (PART 3).

Rising Sea Level.

Along with sea temperature increase, ocean acidification corals are also under threat from the rising sea levels associated with climate change. With a predicted sea level rise (of 6cm/ decade) the potential exists for reefs to ‘drown’ (i.e. be covered with such a depth of water that they are below the photic zone or otherwise connect calcify sufficiently to catch up with sea level), but this requires a protracted imbalance between reef accretion rates and sea level rise (Smith & Buddemeier 1992).  

Depending on water clarity and other environmental conditions, the depth range of maximum reef calcification may extend from several metre s to more than 20 metres. This depth range represents a safety factor; transient sea level rise may inundate oceanic reefs to a depth of metre s or even tens of metre s without terminating reef growth if sea levels  rise subsequently returns to a rate less than reef vertical accretion rates. A reef accretion rate of 10 mm/yr is commonly taken as the consensus value for maximum sustained reef vertical accretion rates (Smith & Buddemeier 1992). The predicted for sea level rise over the next century is on average 6 mm/yr (Smith & Buddemeier 1992); this is well within the range of reef accretion rates, and even with no net accretion sea level rise would submerge reefs by less than a metre  by the year 2100.

On the shorter time scale of years to decades, sea level is a changing environmental variable that may interact with other changes and be reflected in organism and community response. Because sea level has been within 1-2 m of its present elevation for several thousand years, many present-day reefs have grown to an elevation where further upward growth is constrained by sea level. Sea level rise can be expected to remove this constraint and result in increases in successful recruitment and coral longevity on intertidal and shallow subtidal reef flats, with a consequent increase in reef flat calcification (Smith & Buddemeier 1992). If rising sea level creates more benign conditions on shallow reefs, diversity and community structure may change as species other than the extremely hardy are able to survive. Increases in coral community diversity and productivity can also be expected in enclosed lagoons where salinity extremes, nutrient depletion, or other aspects of restricted circulation have restricted reef development (Smith & Buddemeier 1992), since the probable effect of rising sea level on circulation will be to maintain reef/lagoon water composition closer to that of the local oceanic water. On the other hand, if deepening water subjects currently sheltered communities to more physical (wave) stress, calcification and sediment accumulation may not increase.

Sea level rise is also strong associated with coral bleaching (Part 1) and associated mortality may selectively remove faster growing taxa, resulting in less rapid CaCO3 accretion and more rapid net removal of framework material by bioerosion.

References

Smith, S.V. & Buddemeier, R.W. 1992. global change and coral reef ecosystems. Annual review of ecological systems. 23: 89-118.