Effects of Water Column Mixing and Stratification on Planktonic Primary Production and Dinitrogen Fixation on a Northern Red Sea Coral Reef

Abstract

The northern Red Sea experiences strong annual differences in environmental conditions due to its relative high-latitude location for coral reefs. This allows the study of regulatory effects by key environmental parameters (i.e., temperature, inorganic nutrient, and organic matter concentrations) on reef primary production and dinitrogen (N2) fixation, but related knowledge is scarce. Therefore, this study measured environmental parameters, primary production and N2 fixation of phytoplankton groups in the water overlying a coral reef in the Gulf of Aqaba. To this end, we used a comparative approach between mixed and stratified water column scenarios in a full year of seasonal observations. Findings revealed that inorganic nutrient concentrations were significantly higher in the mixed compared to the stratified period. While gross photosynthesis and N2 fixation rates remained similar, net photosynthesis decreased from mixed to stratified period. Net heterotrophic activity of the planktonic community increased significantly during the stratified compared to the mixed period. While inorganic nitrogen (N) availability was correlated with net photosynthesis over the year, N2 fixation only correlated with N availability during the mixed period. This emphasizes the complexity of planktonic trophodynamics in northern Red Sea coral reefs. Comparing mixed and stratified planktonic N2 fixation rates with those of benthic organisms and substrates revealed a close seasonal activity similarity between free-living pelagic and benthic diazotrophs. During the mixed period, N2 fixation potentially contributed up to 3% of planktonic primary production N demand. This contribution increased by ca. one order of magnitude to 21% during the stratified period. Planktonic N2 fixation is likely a significant N source for phytoplankton to maintain high photosynthesis under oligotrophic conditions in coral reefs, especially during stratified conditions.

Phytoplankton Biomass and the Hydrodynamic Regime in NEOM, Red Sea

“…The Gulf of Aqaba appears to reach slightly higher values during the winter peak and slightly lower values during summer, compared to the open waters of the NRS. Although our study area covers only the southern part of the Gulf of Aqaba, its seasonal cycle is in agreement with previous studies that reported a peak during late winter or early spring [72][73][74][75].…”

“…Since there is no significant runoff in the area, these winter blooms can be predominantly attributed to vertical mixing, as the deepening of the mixed layer redistributes nutrients in the water column [80]. When SST becomes lower, surface waters sink and are replaced by deep nutrient-rich waters, whilst strong stratification during summer prevents nutrients from reaching the surface, leading to the isolation of the deeper layer and the accumulation of organic material and nutrients in the near-bottom waters [12,[17][18][19][20]32,47,75]. As previously mentioned, surface Chl-a concentrations in the Gulf of Aqaba remain higher than in the open waters of the NRS during winter.…”

Denitrification Aligns with N2 Fixation in Red Sea Corals

“…Environmental stress may directly affect the equilibrium of these processes, as both eutrophication and the increase in sea surface temperatures directly affect N-cycling within the coral holobiont 14 . Increases in inorganic N availability may lead to a reduction of diazotroph activity in coral holobionts due to the so called “ammonia switch-off” 51 , which is evidenced by negative correlations between N availability and N 2 fixation for both planktonic and benthic diazotrophs 52–54 . Denitrification, on the one hand, may even be stimulated by increased nitrate availability 31 .…”

Relative Diazotroph Abundance in Symbiotic Red Sea Corals Decreases With Water Depth

“…Chlorophyll concentration was the second best environmental variable to explain the relative diazotrophic abundance pattern with depth. While no direct link between ambient chlorophyll concentrations and N 2 fixation is known, the increasing chlorophyll concentration with depth could be due to increased nutrient availability (Tilstra et al, 2018), potentially leading to "ammonia switch-off " (Kessler et al, 2001), and/or increased abundance of zooplankton that graze on phytoplankton. Thus, potentially more N is available for heterotrophic uptake for the corals.…”