On biotic and abiotic drivers of the microphytobenthos seasonal cycle in a temperate intertidal mudflat: a modelling study

Abstract

Microphytobenthos (MPB) from intertidal mudflats are key primary producers at the land-ocean interface. MPB can be more productive than phytoplankton and sustain both benthic and pelagic higher trophic levels. The objective of this study is to assess the contribution of light, mud temperature, and gastropod Peringia ulvae grazing pressure in shaping the seasonal MPB dynamics on the Brouage mudflat (NW France). We use a physical-biological coupled model applied to the sediment 5 first centimeter for the year 2008. The simulated data compare to observations including time-coincident remotely sensed and in situ data. The model suggests a MPB annual cycle characterized by a main spring bloom, a biomass depression in summer, and a moderate fall bloom. In early spring, high simulated photosynthetic rates due to mud surface temperature (MST) values close to the MPB temperature optimum for photosynthesis and to increasing solar irradiance trigger the onset of the MPB spring bloom. After the bloom, high MST values lead to synoptic events when MPB thermo-inhibition (39.5 % of summer) 10 and limitation by P. ulvae grazing (8.7 % of summer) superimpose. During these synoptic events, 14 % of the simulated annual MPB primary production is channeled towards the P. ulvae secondary production through ingestion. The model suggests that such a combined effect is highly linked to the MPB biomass depression in summer. The model ability to infer on biotic and abiotic mechanisms driving the seasonal MPB dynamics could open the door to a new assessment of the export flux of biogenic matter at the land-ocean interface and, more generally, of the contribution of productive intertidal biofilms to the coastal carbon 15 cycle.At temperate latitudes, the seasonal cycle of MPB is shaped by the prevailing environmental conditions. Seasonal blooms are reported to occur throughout the year, i.e. in spring (De Jong and de Jonge, 1995;Sahan et al., 2007; Brito et al., 2013), summer (Cadée and Hegeman, 1977) and fall (Hubas et al., 2006;Garcia-Robledo et al., 2016). Along the French Atlantic coast, the spring bloom and summer depression observed in the Brouage mudflat in the Marennes-Oléron Bay are explained by optimal temperature conditions and thermo-inhibition, respectively (Blanchard et al., 1997). Reported differences in the 5 observed MPB seasonal cycles are also attributed to the diatom assemblage (Underwood, 1994). In terms of biomass, epipelic diatoms associated with muddy sediments show a higher seasonality caused by a marked exposure to stressful environmental conditions (e.g. cycle of deposition/erosion, dessication, grazing) than less motile epipsammic species buried in coarser sandy sediments (Underwood, 1994). In summer, thermo-inhibition and a high grazing pressure by deposit feeders are suggested to dampen the MPB biomass (Cadée and Hegeman, 1974; Cariou-Le Gall and Blanchard, 1995;Sahan et al., 2007). On intertidal 10 mudflats, the prosobranch gastropod Peringia ulvae can reach densities up to 30 000 snails m −2 (Sauriau et al.,...

Impact of Chronic and Massive Resuspension Mechanisms on the Microphytobenthos Dynamics in a Temperate Intertidal Mudflat

“…A total of 10,000 simulations was run using random sets of biological ( F mini , γ ) and physical ( ω , τ baresed , E 0 , z 0 ) constants varying within the reported observed ranges (Table ). This approach was used in Savelli et al (). The constants were chosen as they shape the simulated MPB production and the occurrence and rate of MPB resuspension.…”

“…Consequently, we introduced a new compartment that represented the dynamics of MPB biomass in the fluff layer. Such a compartment was not taken into account in Savelli et al (). The fluff layer was represented by the B compartment (mg Chl a m −2 ), supplemented by a flux of MPB biomass from the biofilm to the fluff layer: where B max is the saturation of Chl a concentration in the fluff layer.…”

“…Considering a mean Chl a concentration in the sediment of 500μg Chl a g −1 of dry sediment and a maximal resuspended mass of sediment of 23 g of dry sediment m −2 (Orvain et al, ), B max was set to 11.5 mg Chl a m −2 . Here, Z was expressed in density of individuals (ind m −2 ) using the same P. ulvae monthly averaged individual weight estimated in Savelli et al (). Therefore, the higher is the MPB biomass in the biofilm ( S ), the higher is the flux of MPB biomass from the biofilm to the fluff layer.…”

“…The model run with the explicit representation of wave‐ and tidal‐induced MPB resuspension (PHY run ) was compared to a reference run (REF run ) with a constant rate of MPB resuspension during high tides without any parametrization of biologically consolidated sediment was used. The setup of the REF run is detailed in Savelli et al ().…”

“…Random values of the bed roughness length ( z 0 ), critical BSS for resuspension of bare sediment ( τ baresed ), consolidation factor ( ω ), erosion constant of pure mud ( E 0 ), minimum MPB biomass in the sediment ( F mini ), and mean time spent by a MPB cell at the sediment surface ( γ ) were selected within the observed ranges (Table ) by a Monte Carlo fixed sampling method (Hammersley & Handscomb, ). The method aims to quantify how simultaneous variations of key biological constants might impact the simulated MPB production and instantaneous resuspension (see Savelli et al, ). A total of 10,000 model runs was performed using the same set of initial conditions.…”