N₂ fixation and primary productivity in a red sea Halophila stipulacea meadow exposed to seasonality

Abstract

The tropical seagrass Halophila stipulacea exhibits a limited capacity to use nitrate, but has a high uptake capacity and efficiency for ammonium. Consequently, N 2 fixation by associated diazotrophic epiphytes may be important in providing ammonium needed for seagrass photosynthesis and growth. When exposed to seasonality, this association could provide a competitive advantage for H. stipulacea in relation to other seagrass species, but related knowledge is scarce. Here, we thus report measurements of net photosynthesis (O 2 fluxes) and N 2 fixation (acetylene reduction) associated with H. stipulacea from the Northern Red Sea over all four seasons of 2013. In parallel, we characterized the seagrass meadow areal extent, shoot and leaves density, and the leaf area index. Using this data, we quantified meadow net primary production and N 2 fixation and estimated the photo-metabolic N demand met by N 2 fixation at the community level. Results revealed a marked seasonality of meadow N 2 fixation, with rates ranging from 0.06 in winter to 4.60 mmol N m 22 d 21 in summer, i.e., an increase by two orders of magnitude. In summer, when nutrient concentrations in the water column were lowest and light intensity and temperature highest, there was a significant positive linear relationship between meadow net primary production and N 2 fixation, with the latter accounting for ca. 20% of the photo-metabolic N demand. These findings suggest that N 2 fixation by associated diazotrophic epiphytes can mitigate N limitation of H. stipulacea meadows under N scarcity. This trait may also favor the ongoing geographical expansion of H. stipulacea.

Chlorophyll–Nutrient Relationships of an Artificial Inland Lagoon Equipped with Seawater Replenishment System in the Northern Red Sea (Gulf of Aqaba)

“…Ayla's UL experiences more widely varying temperatures (16.2-28.8 • C), as shown in Supplementary Files S1 and S2 and discussed by Manasrah [29], than the ambient conditions in the GoA water (22.7-27.7 • C) [18,27,29,[33][34][35][36]. In particular, the UL has much lower winter temperatures (16.2-21.8 • C) than the ambient GoA water conditions (21.9-23.8 • C); this is due to the UL's shallow depth (~2 m) and high surface area, which makes it more vulnerable to losing heat due to surface wind effects, therefore decreasing its temperature during the winter.…”

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

“…Global seagrass are a from [194], macroalgae area from [195] and are a of othe r e cosystems from [188], e xcept the global coastal ocean [193] which e ncompasses estuarie s, we tlands, e stuaries, and contine ntal shelves. Seagrass data [188] update d from [196][197][198][199][200][201][202][203][204][205][206][207][208][209] and macroalgae data from . Plankton GPP and R in salt marshes from [236] and in mangroves from [52].…”

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

“…Global seagrass area from [195], macroalgae area from [196] and area of other ecosystems from [188], except the global coastal ocean [194], which encompasses estuaries, other wetlands, and continental shelves. Seagrass data [188] updated from [197][198][199][200][201][202][203][204][205][206][207][208][209][210] and macroalgae data from . Plankton GPP and R in salt marshes from [237] and in mangroves from [52].…”