Abstract
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 first centimetre for the year 2008. The simulated data compare to observations, including time-coincident remotely sensed and in situ data. The model suggests an MPB annual cycle characterised by a main spring bloom, a biomass depression in summer, and a moderate fall bloom. In early spring, simulated photosynthetic rates are high due to mud surface temperature (MST) values close to the MPB temperature optimum for photosynthesis and because increasing solar irradiance triggers the onset of the MPB spring bloom. Simulated peaks of high P. ulvae grazing (11 days during which ingestion rates exceed the primary production rate) mostly contribute to the decline of the MPB bloom along with the temperature limitation for MPB growth. In late spring–summer, the MPB biomass depression is due to the combined effect of thermo-inhibition and a moderate but sustained grazing pressure. The model ability to infer biotic and abiotic mechanisms driving the seasonal MPB dynamics could open the door to a new assessment of the export flux of biogenic matter from the coast to the open ocean and, more generally, of the contribution of productive intertidal biofilms to the coastal carbon cycle.
Highlights
Coastal and nearshore waters receive large amounts of organic matter and inorganic nutrients from land that support high biological productivity (Mann, 1982; Admiraal, 1984; Hopkinson and Smith, 2005)
This study is a first attempt to simulate the MPB seasonal cycle observed on a temperate intertidal mudflat and to quantify the relative contribution of both biotic and abiotic factors to seasonal MPB dynamics
– In winter and early spring, the seasonal mass-specific maximum photosynthetic rate mainly driven by the simulated mud surface temperature (MST) and the seasonal low C : chlorophyll a (Chl a) ratios lead to a seasonal maximum of MPB growth rate and to an MPB spring bloom
Summary
Coastal and nearshore waters receive large amounts of organic matter and inorganic nutrients from land that support high biological productivity (Mann, 1982; Admiraal, 1984; Hopkinson and Smith, 2005). The high turbidity of estuarine-influenced coastal waters limits the penetration of downward solar irradiance in the water column and, as such, phytoplankton production (Cloern, 1987; Struski and Bacher, 2006). In subtidal and intertidal zones, primary production (PP) sustained by benthic microalgae, or microphytobenthos (MPB), can exceed that of phytoplankton (Underwood and Kromkamp, 1999; Struski and Bacher, 2006). Epipelic MPB are associated with fine cohesive intertidal sediments and develop within the top few millimetres (Underwood, 2001). During daytime exposure, they migrate toward the sediment surface, constituting a dense biofilm of a few hundred micrometres
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