Dynamics of Inorganic Nutrients in Intertidal Sediments: Porewater, Exchangeable, and Intracellular Pools

Abstract

The study of inorganic nutrients dynamics in shallow sediments usually focuses on two main pools: porewater (PW) nutrients and exchangeable (EX) ammonium and phosphate. Recently, it has been found that microphytobenthos (MPB) and other microorganisms can accumulate large amounts of nutrients intracellularly (IC), highlighting the biogeochemical importance of this nutrient pool. Storing nutrients could support the growth of autotrophs when nutrients are not available, and could also provide alternative electron acceptors for dissimilatory processes such as nitrate reduction. Here, we studied the magnitude and relative importance of these three nutrient pools (PW, IC, and EX) and their relation to chlorophylls (used as a proxy for MPB abundance) and organic matter (OM) contents in an intertidal mudflat of Cadiz Bay (Spain). MPB was localized in the first 4 mm of the sediment and showed a clear seasonal pattern; highest chlorophylls content was found during autumn and lowest during spring-summer. The temporal and spatial distribution of nutrients pools and MPB were largely correlated. Ammonium was higher in the IC and EX fractions, representing on average 59 and 37% of the total ammonium pool, respectively. Similarly, phosphate in the IC and EX fractions accounted on average for 40 and 31% of the total phosphate pool, respectively. Nitrate in the PW was low, suggesting low nitrification activity and rapid consumption. Nitrate accumulated in the IC pool during periods of moderate MPB abundance, being up to 66% of the total nitrate pool, whereas it decreased when chlorophyll concentration peaked likely due to a high nitrogen demand. EX-Nitrate accounted for the largest fraction of total sediment nitrate, 66% on average. The distribution of EX-Nitrate was significantly correlated with chlorophyll and OM, which probably indicates a relation of this pool to an increased availability of sites for ionic adsorption. This EX-Nitrate pool could represent an alternative nitrate source with significant concentrations available to the microbial community, deeper in the sediment below the oxic layer.

Water column dissolved silica concentration limits microphytobenthic primary production in intertidal sediments

“…), as has been also observed for nitrate, ammonium, and phosphate (García‐Robledo et al. , , Kamp et al. ).…”

“…Intact sediment cores ( n = 25, internal diameter: 5.4 cm, length: 10 cm) were taken in April 2011 from an intertidal, unvegetated mudflat area within the inner Bay of Cadiz (Trocadero Island, 36°31′09″ N, 6°12′01″ W) at low tide as described in previous studies (García‐Robledo et al. ).…”

“…A recent study performed in an intertidal mud flat (inner Bay of Cádiz, SW Spain) found a positive correlation between net primary production and concentration of DSi in the water column (García‐Robledo et al. ), suggesting that MPB primary production might be controlled by the availability of DSi. Thus, we wanted to test whether water column DSi concentration affects MPB metabolism and fluxes across the sediment water interface in an intertidal mudflat.…”

Satellite remote sensing reveals a positive impact of living oyster reefs on microalgal biofilm development

“…In some northwestern European flats, MPB maxima have been detected slightly later in spring and in September (van der Wal et al, 2010). The summer depression has been observed in some more southerly mudflats such as MarennesOléron Bay (France) (Cariou-Le Gall and Blanchard, 1995), the Tagus estuary (Portugal) (Brito et al, 2013), Cadiz Bay (Spain) (Garcia-Robledo et al, 2016), and in the Wadden Sea (The Netherlands) (van der Wal et al, 2010;Stief et al, 2013), and has been related to increased macrobenthos grazing in summer (Peletier, 1996;Weerman et al, 2011). Low NDVI values in summer may also be due to thermo-inhibition when sediment temperature exceeds 25 • C (Blanchard et al, 1997).…”

“…Via the release of organic and inorganic matter through excretion and biodeposition (Dame, 1993;Cognie and Barillé, 1999;Newell, 2004;Buzin et al, 2015), oysters can have an impact by enriching the sediment around them, increasing nutrient availability and hence development of MPB (Dame, 1993;Garcia-Robledo et al, 2016). Oyster reefs are also known to have indirect effects such as modifying the structure of the surrounding sediment and the ambient hydrodynamic conditions, facilitating MPB establishment (Colden et al, 2016).…”