Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

Abstract

Abstract. Benthic fluxes of dissolved nutrients in reef communities are controlled by oceanographic forcing, including local hydrodynamics and seasonal changes in oceanic nutrient supply. Up to a third of reefs worldwide can be characterized as having circulation that is predominantly tidally forced, yet almost all previous research on reef nutrient fluxes has focused on systems with wave-driven circulation. Fluxes of dissolved nitrogen and phosphorus were measured on a strongly tide-dominated reef platform with a spring tidal range exceeding 8 m. Nutrient fluxes were estimated using a one-dimensional control volume approach, combining flow measurements with modified Eulerian sampling of waters traversing the reef. Measured fluxes were compared to theoretical mass-transfer-limited uptake rates derived from flow speeds. Reef communities released 2.3 mmol m−2 d−1 of nitrate, potentially derived from the remineralization of phytoplankton and dissolved organic nitrogen. Nutrient concentrations and flow speeds varied between the major benthic communities (coral reef and seagrass), resulting in spatial variability in estimated nitrate uptake rates. Rapid changes in flow speed and water depth are key characteristics of tide-dominated reefs, which caused mass-transfer-limited nutrient uptake rates to vary by an order of magnitude on timescales of ∼ minutes–hours. Seasonal nutrient supply was also a strong control on reef mass-transfer-limited uptake rates, and increases in offshore dissolved inorganic nitrogen concentrations during the wet season caused an estimated twofold increase in uptake.

High summer temperatures amplify functional differences between coral‐ and algae‐dominated reef communities

“…They are in stark contrast, however, with the expectation that net autotrophic communities would act as sinks for dissolved inorganic nutrients, as generally measured in single organism incubations (e.g., Den Haan et al 2016). Although a theoretical N requirement of 34–83 mmol N·m −2 ·d −1 to support NCP can be expected (stoichiometric calculations with NCP ranging from 230 to 550 mmol C·m −2 ·d −1 ; assuming a C/N ratio of 6.6; see Redfield 1958), the effects of assimilation were likely masked by concurrent community‐wide processes that produce DIN (Gruber et al 2019). For example, cavities within Red Sea reefs can be considerable sources of DIN (>20 mmol N·m −2 ·d −1 ) as sponges and other filter feeders utilize dissolved organic matter (Richter et al 2001).…”

Wave and Tidally Driven Flow Dynamics Within a Coral Reef Atoll off Northwestern Australia

“…For wave‐driven reefs, wave forces (radiation stress gradients) generated by breaking waves increase the mean water level (wave setup) in the surf zone, establishing pressure gradients that drive flow across reef flats and into a lagoon, with water eventually returning back to the ocean through channels in the reef (Figures 1a and 1b; Gourlay, 1996; Lowe et al., 2009a; Munk & Sargent, 1954; Symonds et al., 1995; Young, 1989). For tidally driven reefs, tides create sea level gradients between the reef and offshore that drive oscillatory flows in and out of a reef over a tidal cycle (Figures 1a, 1c and 1d; Dumas et al., 2012; Green et al., 2018; Gruber et al., 2019; Lowe et al., 2015).…”

“…The circulation of water within reef systems and water exchange with the surrounding ocean regulates reef environmental conditions (e.g., temperature variability) and the transport of material within and between reefs (e.g., coral larvae, nutrients, and sediment; Falter et al., 2004; Gruber et al., 2019; Lugo‐Fernández et al., 2001; Storlazzi et al., 2004). Reef circulation is generally driven by a combination of oceanic and atmospheric processes including waves; tides; wind; buoyancy; and regional‐scale ocean circulation (Lowe & Falter, 2015; Monismith, 2007), with the relative importance of each mechanism varying by the conditions at a site and scale of the flows considered (Monismith, 2007).…”

Turbulence and Coral Reefs

“…Likewise, the single-colony calculations of Chang et al (2014) suggest that the relationship between drag (primarily associated with separation and thus pressure forces) and mass transfer (associated with near-wall shear and strain) may not be robust. Nonetheless, this formalism has been used with some success to estimate mass transfer-limited nutrient fluxes on reefs other than Kaneohe Bay (Falter et al 2012, Wyatt et al 2012, Gruber et al 2019.…”