Spatial variability in macrofaunal diet composition and grazing pressure on microphytobenthos in intertidal areas

Abstract

Microphytobenthos forms an important part of the diet of macrofauna (macrozoobenthos) in many intertidal ecosystems. It is unclear, however, whether the dependence of macrofauna on microphytobenthos varies spatially within and among tidal systems. We aim (1) to assess the spatial variability in the importance of microphytobenthos in the diet of macrofauna (i.e., between and within two tidal basins and as function of elevation), (2) to quantify grazing pressure of the macrofaunal community on different potential food sources (microphytobenthos, phytoplankton and terrestrial organic material) for several sites in two tidal basins and (3) to compare microphytobenthic production and summer/autumn grazing of the total macrofaunal community and grazing pressure per feeding type, with potential microphytobenthic production estimated from rates in early spring, when grazing was low. Using a natural stable isotope approach, we identified microphytobenthos as a more important food source for macrofauna than phytoplankton and terrestrial organic material. Microphytobenthos dependency differed between tidal basins for the genera Bathyporeia (sand digger shrimp), Macoma (Baltic tellin), and Peringia (mudsnail) and for sampled individuals of all genera combined, and did not vary as function of elevation. We showed that macrofaunal grazing on microphytobenthos is quantitatively important and, in some cases, approached microphytobenthic production rates in early spring. No positive relation between microphytobenthic production in early spring and macrofaunal grazing in summer/autumn was observed. This suggests that the studied consumer-resource interactions are coupled on a larger spatial scale (i.e., mesoscale, ≈10 to 100 km), rather than the fine (mm to m) scale. Understanding food webs is highly relevant for assessments of the impact of environmental changes (e.g., eutrophication, erosion, or sea level rise) on aquatic ecosystems (Middelburg 2014). Benthic primary producers or microphytobenthos (microphytobenthos), consisting of unicellular eukaryotic algae and cyanobacteria, can form a significant part of total estuarine primary production (Underwood and Kromkamp 1999). Microphytobenthos provides an important source of organic material for benthic micro-, meio-, and macrofauna on intertidal flats

Taxonomic and Functional Patterns of Benthic Communities in Southern Temperate Tidal Flats

“…Deposit feeders are generally dominant in muddy sediments (Rhoads and Young, 1970;Hajializadeh et al, 2020), and the sediment grain size (D50 and sorting) was smaller in summer compared to winter. The deposit feeders and grazer trait modalities were also expressed most at sites where high concentrations of Chl a were found, as they feed on microphytobenthos (e.g., Wong and Dowd, 2015;Daggers et al, 2020). The relationship of different feeding mode modalities with sediment conditions and primary productivity could also indicate differentiation in the use of resources, food availability, and prey accessibility across sites and seasons (Norkko et al, 2013;Weigel et al, 2016;Sivadas et al, 2020).…”

Quantifying the role of microphytobenthos in temperate intertidal ecosystems using optical remote sensing

“…The δ 13 C of the fatty acids was determined using a Varian 3400 gas chromatograph containing a Varian SPI injector, which was coupled to a Finnigan Delta S isotope ratio mass spectrometer (Middelburg et al, 2000). The δ 13 C of the entire diatom cell was determined using fractionation factors of +6.5 (20:5w3) and +7.5 (22:6w3) as described in Daggers et al (2020).…”

“…Sediment samples were freeze dried/ stored frozen and subsequently acidified to remove inorganic carbonates (Nieuwenhuize et al, 1994) and measured for the δ 13 C and δ 15 N using the mass spectrometer. The δ15N values of diatoms were estimated using the δ15N of the sampled bulk sediment +2 for the Oosterschelde and +10 for the Westerschelde, using the difference between the δ15N of sedimented organic matter and benthic algae as reported by Riera et al (2000) (Daggers et al, 2020). Water samples for the determination of the natural stable isotope composition of Suspended Particulate Organic Matter (SPOM) were collected at T8 and T18/T19 and used as indicator for phytoplankton (Galvan et al, 2008;Evrard et al, 2010;Daggers et al, 2020).…”

“…The δ15N values of diatoms were estimated using the δ15N of the sampled bulk sediment +2 for the Oosterschelde and +10 for the Westerschelde, using the difference between the δ15N of sedimented organic matter and benthic algae as reported by Riera et al (2000) (Daggers et al, 2020). Water samples for the determination of the natural stable isotope composition of Suspended Particulate Organic Matter (SPOM) were collected at T8 and T18/T19 and used as indicator for phytoplankton (Galvan et al, 2008;Evrard et al, 2010;Daggers et al, 2020). Earlier studies have demonstrated that in summer, SPOM in the Westerschelde is dominated by autochtonous phytoplankton (Mariotti et al, 1984).…”

“…Diatoms often dominate the MPB of intertidal sediments and contribute significantly to estuarine primary production (Underwood and Kromkamp, 1999). Benthic diatoms produce extracellular polymeric substances (EPS) that reduce sediment erosion (Yallop et al, 1994) and were found to be a major food source for part of the macrofauna living in intertidal areas (Christianen et al, 2017;Daggers et al, 2020;Page, 1997).…”