EFFECT OF MANGROVE DETRITAL OUTWELLING ON NUTRIENT REGENERATION AND OXYGEN FLUXES IN COASTAL SEDIMENTS OF THE CENTRAL GREAT-BARRIER-REEF LAGOON

The effect of tidal outwelling of mangrove detritus on sediment nutrient chemistry, nutrient regeneration and oxygen fluxes in a coastal area of the central Great Barrier Reef lagoon was examined. Organic carbon and total nitrogen concentrations ranged from 0·2 to 3·9% and 0·01 to 0·18% by sediment dry weight (DW), respectively, and were highest at stations receiving the greatest quantities of mangrove litter. Total phosphorus concentrations ranged from 0·013 to 0·048% by DW, but did not relate to outwelling. C:N:P. ratios ranged from 29:6:1 at the site receiving the least amount of detritus to a high of 397:17:1 at the station receiving the most litter. The effect of outwelling on dissolved inorganic nutrients in porewaters was less clear, with concentrations closely related to sediment type and obscured by contributions from continental runoff. Ammonium (range: 10-310 μM) and silicate (range: 10-315 μM) were in greatest concentration at most sites followed by phosphate (range: 1-12 μM) and nitrite and nitrate (0·07-6·0 μM). Directly measured fluxes of dissolved inorganic nutrients across the sediment-water interface were either undetectable or small [ΣN = -560 to +3503; PO43 = undetectable to +157; Si(OH)4 = undetectable to +6449 μmol m-2 day-1] and not related to outwelling. Addition of mangrove litter to boxcore samples incubated for 10 days in the laboratory did not affect rates of dissolved inorganic nutrient release. Oxygen consumption rates ranged from 8·7 to 60·2 mmol O2 m-2 day-1 and were highest at most sites closest to mangrove forests, in agreement with previous measurements of bacterial productivity. Rates of net community primary production were either undetectable or low (as were chlorophyll α and phaeopigments) ranging from 12 to 77 mg C m-2 day-1. It appears that much of the deposited mangrove litter is highly refractory, but the amount of sedimented material appears to be great enough to significantly enrich bulk concentrations of particulate carbon and nitrogen, and rates of oxygen consumption. Benthic nutrient regeneration contributes only a small proportion to the daily nitrogen (6%) and phosphorus (9%) demand of coastal phytoplankton, implying supplementation from continental runoff and/or high rates of pelagic recycling. © 1990.