Benthic uptake of phytoplankton and ocean‐reef exchange of particulate nutrients on a tide‐dominated reef

Abstract

Benthic fluxes of chlorophyll a (Chl a) and particulate organic carbon (POC) and nitrogen (PON) were quantified on Tallon reef, a strongly tide-dominated (spring range > 8 m) reef located in the Kimberley region of northwestern Australia, over a 2-week period. Extensive hydrodynamic observations were used to construct a reef-scale mass balance to estimate material exchange between the reef and ocean over individual tidal cycles. Additionally, a one-dimensional control volume approach was used to estimate fluxes of Chl a in waters traversing the reef platform. Particulate material was delivered to the reef platform in a pulse during flood tide, and benthic uptake of Chl a declined to negligible values toward the end of ebb tide. On the scale of tidal cycles, a net uptake of Chl a was observed on the reef platform (on average 1.3 mg Chl a m 22 d 21), which was lower than previous studies of many reef communities. Fluxes showed variability depending on the magnitude of individual tidal cycles, which was likely related to volumes of oceanic Chl a inputs. Tallon reef was a net source of detrital POC and PON to the surrounding coastal ocean, with average POC

Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

“…The Kimberley region shares similar rainfall patterns, tidal ranges, and low levels of catchment alteration with the northern GBR (at a similar latitude to the Kimberley), yet concentrations of DIN and DIP measured in this study were an order of magnitude greater than those from the wet tropics (Furnas et al, 2005;Schaffelke et al, 2012). These observations, coupled with elevated concentrations of chlorophyll a and particulate nutrients (Gruber et al, 2018) relative to 'typical' oligotrophic reef waters, suggest that some coastal Kimberley reefs may experience naturally mesotrophic conditions.…”

“…Tallon reef platform is well-suited to a one-dimensional CoVo approach due to long periods (approximately 10 h of each semidiurnal tidal cycle) of consistent flow direction; nutrient sampling may thus be conducted at 'upstream' and 'downstream' sites during these periods. A similar approach has previously been used on Tallon reef to estimate its benthic metabolism (Gruber et al, 2017) and particulate material uptake (Gruber et al, 2018) rates. A bottom-mounted acoustic Doppler current profiler (Nortek Aquadopp HR) was stationed near SG ( Figure 1) and measured current velocity and water depth (h) at 1 Hz and 0.03 m bins.…”

“…Previous work has attributed inorganic nutrient release to remineralization of particulate material by benthic filter-feeders (Ribes et al, 2005;Wyatt et al, 2012) and detritivores (Silverman et al, 2012), which can graze PON on the order of DIN release rates, as well as nitrification by sponge communities (Southwell et al, 2008). In the case of Tallon reef, uptake of phytoplankton (0.95 mmol N and 0.20 mmol P m -2 d -1 ) (Gruber et al, 2018) is on the order of J release in the case of P, but is much smaller than J release of N. Large particles (such as entire fronds of macroalgae) are rare but can form a major component of the particulate organic pool on some reefs (Alldredge et al, 2013); remineralisation of similar material (rather than small particles like phytoplankton) may be the source of the observed DIN release on Tallon. Finally, fluxes of DON on the order of J net were measured on Tallon, with net uptake occurring during the Feb experiment (Figure 4d).…”

“…Extended periods of low water depth on reef platforms such as Tallon Island can cause communities to experience high irradiances that result in diel temperature changes up to 11° C (Lowe et al, 2016) and dissolved oxygen fluctuations among the most extreme measured worldwide (Gruber et al, 2017). Recent measurements of coral calcification (Dandan et al, 2015), seagrass productivity (Pedersen et al, 2016), reef community metabolism (Gruber et al, 2017), and particulate nutrient uptake (Gruber et al, 2018) have been published from tide-dominated systems, yet little is currently known about how these large tides control fluxes of dissolved nutrients. The objectives of this study were to: 1) measure fluxes of dissolved N and P on a tidally-forced reef, 2) compare measured rates to maximum potential uptake predicted by mass-transfer theory, and 3) compare tidal forcing (velocity and water depth changes) and oceanic forcing (seasonal changes in nutrient concentration) of mass-transfer-limited uptake rates.…”

Tidal and seasonal forcing of dissolved nutrient fluxes in reef communities

“…4d). The dynamics of DON in reef systems have been addressed in a few studies (e.g., Haas and Wild, 2010;Thibodeau et al, 2013;Ziegler and Benner, 1999), and there is some evidence that reef organisms including corals (Ferrier, 1991), sponges (Rix et al, 2017), and seagrasses (Vonk et al, 2008) can directly utilize DON. In summary, gross release of DIP may be derived from phytoplankton uptake on Tallon reef, but released DIN exceeds phytoplankton inputs and is likely derived from additional sources including remineralization of large particles and DON.…”