Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea

Abstract

While various sources increasingly release nutrients to the Red Sea, knowledge about their effects on benthic coral reef communities is scarce. Here, we provide the first comparative assessment of the response of all major benthic groups (hard and soft corals, turf algae and reef sands—together accounting for 80% of the benthic reef community) to in-situ eutrophication in a central Red Sea coral reef. For 8 weeks, dissolved inorganic nitrogen (DIN) concentrations were experimentally increased 3-fold above environmental background concentrations around natural benthic reef communities using a slow release fertilizer with 15% total nitrogen (N) content. We investigated which major functional groups took up the available N, and how this changed organic carbon (Corg) and N contents using elemental and stable isotope measurements. Findings revealed that hard corals (in their tissue), soft corals and turf algae incorporated fertilizer N as indicated by significant increases in δ15N by 8%, 27% and 28%, respectively. Among the investigated groups, Corg content significantly increased in sediments (+24%) and in turf algae (+33%). Altogether, this suggests that among the benthic organisms only turf algae were limited by N availability and thus benefited most from N addition. Thereby, based on higher Corg content, turf algae potentially gained competitive advantage over, for example, hard corals. Local management should, thus, particularly address DIN eutrophication by coastal development and consider the role of turf algae as potential bioindicator for eutrophication.

Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

“…Nevertheless, a higher N availability (i.e., higher fixed N inputs with stable denitrification activity) in algae-dominated reefs could have multiple consequences resulting in a positive feedback loop. Just as eutrophication promotes turf algae growth on coral reefs 112 , higher N accumulation from N 2 fixation could relieve N limitation and cause algae to proliferate in nutrient-poor waters 113 , 114 . This can result in high abundances of benthic algae that in turn deter herbivorous fish that successively control algal proliferation by grazing 115 .…”

“…Briefly, COBRA incubations were performed in gastight 1 L glass chambers (800 mL seawater + 200 mL headspace). As acetylene inhibits the production of NO 3 - via nitrification 141 , 142 , seawater (ambient NO 3 - concentrations ranged between 0.09 and 0.34 µM) 21 , 112 was supplemented with nitrate to a final concentration of 5 µM as a substrate for the denitrification pathway to counteract substrate limitation (see supplementary material SM 5 ). Incubations with nitrate amended seawater have been performed successfully in previous studies 141 – 145 .…”

Effects of Ocean Acidification on Carbon and Nitrogen Fixation in the Hermatypic Coral Galaxea fascicularis

“…Conversely, N-deficient corals showed reduced metabolism and nutrient limitation, leading to reduced calcification rates and decreased symbiont growth and density (Wiedenmann et al, 2013;Ezzat et al, 2015). The combined reduction of DDN assimilation rates and δ 15 N natural isotope abundance under OA in G. fascicularis (Figure 2) potentially may have compromised the coral calcification, although interpreting this in symbiotic corals is difficult because calcification is affected by various processes (Karcher et al, 2020).…”

Unexpected role of communities colonizing dead coral substrate in the calcification of coral reefs

“…Algal assemblages are usually the first colonizers of bare CaCO 3 substrate and, therefore, are becoming more abundant in coral reef ecosystems as bleaching becomes more frequent and intense and reefs degrade (Rogers and Miller 2006; Hughes et al 2007; Swierts and Vermeij 2016). On top of ocean warming, other anthropogenic pressures such as eutrophication and decreases in density of herbivorous fishes due to fishing pressure can alter competition between corals and algae in ways that favor algal community growth (Mumby et al 2006; Hughes et al 2007; Karcher et al 2020). Understanding how communities growing over dead coral substrate influence coral reef carbonate budgets is crucial to predict whether these ecosystems will be able to maintain net CaCO 3 accretion under future environmental change.…”