In aquarium experiments with the symbiotic reef coral
Diploria strigosa, dissolved organic carbon (DOC) and
nitrogen (DON) concentrations were sampled over a
daylight period. Production of DOC and DON occurred
in several abrupt “pulse events” with concentration
increases of 28 to 133 micro molar carbon and 34 to 83
micromolar nitrogen. Additionally, there was a residual
accumulation in the background concentration of 43 to
165 micromolar DOC and 13 to 52 micromolar DON.
DOM production was compared to the net drawdown of
dissolved inorganic carbon (DIC) and the incorporation
of isotopically labelled bicarbonate (H13CO3) and
nitrate (15NO3) into the coral and zooxanthellae tissue.
Released DOC may be equivalent to as much as 38%
of the net DIC use. In comparison, the carbon
incorporated into the tissue is equivalent to only about
5% of the DIC use.

Results of experiments show that released dissolved
organic matter (DOM) is rich in nitrogen, with C: N ratios
approaching 3:1; this nitrogen-rich material has the
potential to be an important source of carbon and
nitrogen to the reef community. Further observations
have shown daily accumulations of 15 to 30 micromolar
urea carbon in the aquaria. Measurements of urea
concentration on the Bermuda platform display clear
seasonal periodicity, with concentrations increasing
from spring through late summer. Extrapolating rates of
DOM production for Diploria strigosa from aquarium
experiments over the entire Bermuda reef platform, I
demonstrate that coral-zooxanthellae symbioses could
potentially be a dominant part of the seasonal urea
cycle. Additional supporting evidence shows the
seasonal peak in urea concentration coincides with
peaks in ecosystem organic matter production and
annual temperature.