Enewetak (Eniwetok) Atoll: Aspects of the nitrogen cycle on a coral reef1

Abstract

Changes in the concentrations of dissolved and particulate nitrogen in seawater crossing two interisland reef transects on Enewetak Atoll were measured. The upstream half of transect II consisted largely of algal pavement; the downstream half was visually dominated by corals. The other community, transect III, consisted of benthic algal covered pavement in its entirety, with only scattered small corals downstream. Both transects showed a significant net export of combined nitrogen, implying a large input of nitrogen into the reef system from a source other than the combined nitrogen in overlying waters. High rates of gaseous nitrogen fixation were found.There was a net uptake of nitrate by the community dominated by algae and a net export of nitrate from the transect dominatecl jointly by algae and corals. Although nitrification is of central importance in the marine nitrogen cycle, this coral-algal community at Encwetak armears to have been the first marine site at which this process has been mcasured dire&i;.

Down under the tunic: bacterial biodiversity hotspots and widespread ammonia-oxidizing archaea in coral reef ascidians

“…In particular, sequences affiliated with the genus Nitrosopumilus dominated the archaeal communities in GBR ascidians and several Nitrosopumilus OTUs exhibited a widespread distribution among hosts and high relative abundance within hosts. In coral reef waters, observations of high nitrite/nitrate concentrations compared to adjacent, open water habitats have long suggested active nitrification among reefassociated microbes (Webb et al, 1975). More recent studies have reported that host-associated microbes in sponges and corals contributed to nitrification in these reef habitats to a larger extent than reported for free-living communities in sediments and seawater (Diaz and Ward, 1997;Southwell et al, 2008).…”

Oceanographic forcing of nutrient uptake and release over a fringing coral reef

“…unpubl.). Early atoll studies also documented N : P in incoming waters of 2 : 1 (Johannes et al 1972;Webb et al 1975), indicating that most reef systems are supplied with extremely N-poor waters, which makes them that much more N-limited (Odum and Odum 1955). Atkinson and Falter (2003) suggested that this missing nitrogen must come from either POM or nitrogen fixation.…”

“…Changes in dissolved inorganic nutrient concentrations have been examined in a number of coral reef systems (Webb et al 1975;Crossland et al 1984;Miyajima et al 2007a); however, it has been difficult to evaluate how these ecosystem-scale changes reflect fluxes into and out of the individual communities that comprise the larger reef system. Comparatively few studies have quantified fluxes within natural reefs systems at the community scale (Crossland and Barnes 1983;Baird et al 2004;Cuet et al 2011a), largely due to the fact that uptake rates are often too slow to detect over smaller spatial scales (Atkinson and Smith 1987;Atkinson and Bilger 1992 Atkinson and Falter 2003).…”

“…Many early studies of reef-level nutrient dynamics considered the so-called 'coral reef paradox'-the existence of highly productive benthic coral ecosystems in a lownutrient oceanic 'desert'-but likely underestimated the magnitude of net nutrient uptake and, therefore, the importance of oceanic (allochthonous) nutrients (Odum and Odum 1955;Johannes et al 1972;Webb et al 1975). Mass transfer limitation (MTL) theory explains how the formation of a diffusive boundary layer physically limits the rate of nutrient uptake from the overlying water column Atkinson and Bilger 1992;Bilger and Atkinson 1992).…”

Evidence of Large-Scale Chronic Eutrophication in the Great Barrier Reef: Quantification of Chlorophyll a Thresholds for Sustaining Coral Reef Communities

“…Unimpacted coral reef communities exhibit relatively high gross productivity even though the surrounding waters contain low concentrations of nutrients; the net import of N and P is usually low to negligible and, in the case of N is often negative due to N-fixation (e.g., see Webb et al 1975;). The maintenance of this high gross productivity therefore requires high rates of supply of recycled nutrients.…”