Ammonium distributions, production, and efflux from backreef sediments, St. Croix, US Virgin Islands

Abstract

Sediments in < 3 m water depth on Tague Bay backreef, St. Croix, US Virgin Islands, were sampled for grain size, Eh, pH, organic carbon, pore-water ammonium and nitrate + nitrite concentrations, ammonium production, and ammonium and nitrate + nitrite benthic fluxes. Most sampling sites had nutrient-poor, oxidized, coarse sand-sized sediments. Ammonium and nitrate + nitrite concentrations were typically < 5 PM; organic carbon was < 0.6 % g dry weight. High spatial heterogeneity on the scale of cm2 existed in all parameters measured except grain size, which decreased regularly away from the reef. We suggest that the nutrient-poor conditions were maintained by high turbulence in the environment, which would minimize organic input to the sediments and maximize resuspension and oxygenation. This view was supported by ammonification rates (mean = 70 pm01 (1 sediment)-' d-l) and benthic nitrogen fluxes which were low compared to rates reported from many temperate coastal areas. Benthic nitrogen fluxes were highly variable and averaged ?l f 95 (mean + SD) w o l ammonium m-* d-I and -5 f 40 pm01 nitrate + nitrite m-2 d-l. Local pockets of reducing sediments with ammonium concentrations > 30 occurred within the generally low-nutrient environment. Their location was correlated in part to stabilization of sediments by certain benthic microalgae. These areas may be zones of enhanced nitrogen remineralization, benthic nitrogen f l u e s , and possibly nitrification coupled to denitrification. Comparison of the range of fluxes predicted from ammonification data to the range of measured fluxes showed both positive and negative deviations. Nitrification coupled to denitrification cannot be ruled out as an explanation for measured fluxes which were lower than the production-predicted f l u e s because Eh and pH conditions were within the environmental limits of denitrifying organisms. Loss of nitrogen via denitrification would be an important process to verify in coral reef ecosystems where conservation of nitrogen apparently is otherwise maximized.

Porewater nutrient profiles and nutrient sediment–water exchange in a tropical mangrove waterway, Mapopwe Creek, Chwaka Bay, Zanzibar

“…Inconsistent patterns in the intensity and direction of nutrient flux rates and directions in estuarine and mangrove systems have been reported elsewhere (e.g. Williams, Yarish & Gill, 1985; Alongi quoted in Alongi et al ., 1992) and are generally taken to reflect the complex relationships between numerous physico‐chemical and biological control factors as well as the variability of the environment over a small distance scale common to mangrove systems (Callender & Hammond, 1982).…”

Nitrogenase activity in tropical carbonate marine sediments

“…Williams et al (1985) estimated ammonification in homogenized reef sediments of St. Croix (US. Virgin Islands) and found an average net NH: production rate of 70 pm01 (1 sed.…”

Surge ammonium uptake in macroalgae from a coral atoll

“…As far as corals themselves are concerned, the high production could be maintained by a tight recycling of N between the coral and its zooxanthellae (Wafar et al, 1985). Most other autotrophs would have to depend on sources such as upwelling near the reef (Andrews and Gentien, 1982), endogenous upwelling within the reef frame (Rougerie and Wauthy, 1993), ground water seepage (Naim, 1993) and regeneration in sediments (Williams et al, 1985), besides nitrification in a range of animal-bacterial associations (Wafar et al, 1990;Corredor et al, 1988;Welsh and Castadelli, 2004). Besides these 'steady' fluxes, sporadic supply is also important for N economy of the reefs.…”