Assessing biomass and primary production of microphytobenthos in depositional coastal systems using spectral information

Abstract

In depositional intertidal coastal systems, primary production is dominated by benthic microalgae (microphytobenthos) inhabiting the mudflats. This benthic productivity is supporting secondary production and supplying important services to humans including food provisioning. Increased frequencies of extreme events in weather (such as heatwaves, storm surges and cloudbursts) are expected to strongly impact the spatiotemporal dynamics of the microphytobenthos and subsequently their contribution to coastal food webs. Within north-western Europe, the years 2018 and 2019 were characterized by record-breaking summer temperatures and accompanying droughts. Field-calibrated satellite data (Sentinel 2) were used to quantify the seasonal dynamics of microphytobenthos biomass and production at an unprecedented spatial and temporal resolution during these years. We demonstrate that the Normalized Difference Vegetation Index (NDVI) should be used with caution in depositional coastal intertidal systems, because it may reflect import of remains of allochthonous pelagic productivity rather than local benthic biomass. We show that the reduction in summer biomass of the benthic microalgae cannot be explained by grazing but was most probably due to the high temperatures. The fivefold increase in salinity from January to September 2018, resulting from reduced river run-off during this exceptionally dry year, cannot have been without consequences for the vitality of the microphytobenthos community and its resistance to wind stress and cloud bursts. Comparison to historical information revealed that primary productivity of microphytobenthos may vary at least fivefold due to variations in environmental conditions. Therefore, ongoing changes in environmental conditions and especially extreme events because of climate change will not only lead to changes in spatiotemporal patterns of benthic primary production but also to changes in biodiversity of life under water and ecosystem services including food supply. Satellite MPB data allows for adequate choices in selecting coastal biodiversity conservation and coastal food supply.

Spatial and temporal patterns of microphytobenthos communities along the marine-terrestrial boundary in the German Wadden Sea

“…The significant increase in MPB biomass with elevation in the tidal flats matched the findings of previous studies of other European tidal flats (Brotas et al, 1995;van der Wal et al, 2010;Daggers et al, 2020). Upper tidal flats fall dry for a longer period of time during low tide, resulting in higher stability (low resuspension rate) of the sediment, and longer exposure to sunlight, which therefore increases the photosynthetic period (van der Wal et al, 2017;Jacobs et al, 2021). We did not find a significant relationship between ENS and elevation, which is in contrast to previous studies that observed a decrease in MPB diversity with increasing elevation, driven by local in-situ factors, such as sediment grain size and mud content (Scholz and Liebezeit, 2012b;Ribeiro et al, 2021).…”

“…We did not find a significant relationship between ENS and elevation, which is in contrast to previous studies that observed a decrease in MPB diversity with increasing elevation, driven by local in-situ factors, such as sediment grain size and mud content (Scholz and Liebezeit, 2012b;Ribeiro et al, 2021). Other studies related the opposite pattern (increasing diversity with elevation) to stronger wave action and hydrodynamic conditions at the seaward side of the elevation gradient, potentially causing resuspension of MPB, and the mixing of sediments and thus burying MPB cells into deeper sediment layers (Underwood and Kromkamp, 1999;van der Wal et al, 2010;Jacobs et al, 2021). Janousek (2009) also reported higher pigment diversity in the salt marsh compared to the adjacent bare tidal flats, due to the various conditions provided by salt marsh vegetation already described earlier.…”

“…Additionally, factors, such as sediment input and bathymetry can also impact sedimentation rates (Morris et al, 2002;Fagherazzi et al, 2014). In general, sediment grain size decreases with increasing elevation due to reduced wave action, which allows for fine particles like silt and clay to settle close to the salt marshes (Brotas et al, 1995;van der Wal et al, 2017;Jacobs et al, 2021). Finer muddier sediments are associated with higher organic matter, water, and nutrient content, which support higher MPB biomass (Pomeroy, 1959;Brotas et al, 1995;Barranguet et al, 1998;Thornton et al, 2002).…”

“…Algal biomass tends to be higher on the upper tidal flats due to longer emersion duration and shallower water depth during high tide, which results in higher irradiances, warmer temperatures, and higher sediment stability (low resuspension rate) (van der Wal et al, 2010(van der Wal et al, , 2017Daggers et al, 2020;Jacobs et al, 2021). Algal biomass is further strongly influenced by sediment properties, which affect water content, organic matter content, and light penetration.…”

Rhamnolipids Mediate the Effects of a Gastropod Grazer in Regards to Carbon–Nitrogen Stoichiometry of Intertidal Microbial Biofilms

“…Diatoms can break down and recycle environmental nutrients, such as nitrogen, which is necessary in typically nitrogen-limited marine environments [8,9]. Diatoms are also very important autotrophs, acting as major primary producers in marine environments [10,11].…”