SEDIMENT OXYGEN DEMAND AND STOICHIOMETRY OF IRL NUTRIENT FLUXES

Abstract

Eutrophication of coastal systems has become a prominent global issue with a variety of detrimental effects, one of these being the shift in the composition of phytoplankton communities often promoting harmful algal blooms (HABs) resulting at least partially from changes to the relative abundance of N:P:Si. This study investigated nutrient fluxes from muck, a fine-grained and organic-rich sediment, and from sand to determine the importance of these sediments with regards to regulating N:P ratios in the water column. Overall sediment oxygen demand (SOD) was used as an indicator of aerobic decomposition and was found to decrease from north to south following the equation: SOD = -178[decimal degrees] +4934(p-value=0.0015). Consistent with high rates of SOD in muck, very high fluxes of ammonium (310 ± 342 μmol N/m2/hr) and phosphate (20.1 ± 31.8 μmol P/m2/hr) were identified for muck sediments. In anaerobic muck, nitrate was consumed as an oxidizing agent (reduced to ammonium) and muck was identified as a sink for nitrate+nitrite with a mean flux of -111 ± 204 μmol N/m2/hr. As a source of ammonium and a sink for nitrate, the flux of readily bioavailable dissolved inorganic nitrogen (DIN) from muck averaged ~200 μmol N/m2/hr. In contrast, ammonium fluxes from sand were on average >5-times lower (56.0 ± 77.3 μmol N/m2/hr) than fluxes from muck and phosphate fluxes were directed into sediments (-7.6 ± 8.9 μmol P/m2/hr) with sand acting as a sink for phosphate. Sandy sediments likely promoted the nitrification of ammonium to nitrate with an efflux of nitrate+nitrite of 55 ± 219 μmol/m2/hr. Overall, sand was a source of nitrate and ammonium with a net DIN flux of 110 μmol/m2/hr. Based on this data, no statistically significant correlations were identified for molar DIN:DIP flux ratios versus organic matter (OM) content or seepage. Although there was no significant difference between the ratio of readily bioavailable DIN:DIP fluxes (ammonium + nitrate + nitrite over phosphate) of sand (9.9 ± 27.2) versus muck (16.6 ± 29.1), the mean total dissolved nitrogen to total dissolve phosphate (TDN:TDP) flux ratio of sand (-28.6 ± 54.4) was significantly different than that of muck (50.6 ± 87.6). Results of this study indicated that muck deposits promote relatively high TDN:TDP ratios consistent with those preferred by HAB species Pyrodinium bahamense var. bahamense, Aureoumbra Lagunensisand Akashiwo sanguinea experienced by the northern IRL where muck is prominent. Overall, the difference in nutrient fluxes experienced by sand and muck are indicative of muck acting in detriment to water quality by creating a positive feedback loop of eutrophication while sand can buffer against eutrophication and can promote the growth of beneficial photosynthesizers such as seagrasses.