Microscale Variability in Biomass and Photosynthetic Activity of Microphytobenthos During a Spring-Neap Tidal Cycle

Abstract

Carbon fixation by microphytobenthic algae of intertidal flats often dominates the total primary production of turbid, temperate estuaries. Whilst remote sensing can accurately measure the spatial distribution of photosynthetic biofilms at the mesoscale (1-300 m), variability at smaller scales requires in-situ investigation. Here, changes in biomass and photosynthetic activity of microphytobenthos (MPB) at the micro-scale (<1 m) were investigated. Biomass of MPB was estimated from repeated high resolution spectral measurements carried out at intervals along a short transect. Whilst the mean concentration of MPB remained constant over time, considerable variability in the spatial and temporal dimensions was measured. The biofilm grew and diminished along the transect over time, and net growth and loss rates could be established under natural conditions. Daily vertical migration was observed for the first time in an undisturbed sediment, and was modeled as a function of solar elevation and tidal angle. Top down factors such as grazing, or physical disturbance, played an important role in the changes in biomass. Photosynthetic activity was measured with portable fluorometers: one device used an artificial light source, the other measured the effective photosystem-II quantum efficiency (F/F m) in-situ with solar irradiance. No evidence was found for photoinhibition, with F v /F m values consistently high, indicating an ability of the diatom biofilm to remain productive under periods of high irradiance. The maximum rate of photosynthesis reached with artificial light was rarely achieved in-situ, even with full solar exposure. There was, however, a very good agreement between the predicted rate at ambient irradiance, and that actually measured. Composite photosynthesis-irradiance curves over the whole experimental period were similar, and showed the same initial slopes, but the maximum rate of relative photosynthetic electron transport (rETR m) based on the in-situ measurement was lower. When total depth-averaged primary production during low tide was calculated, the differences between methods varied between 5 and 13%, and were smallest when surface irradiance values were highest. These results show that the protocol used to measure the photosynthetic activity of the MPB had only minor importance on the overall productivity estimate compared to accurate knowledge of photosynthetically active biomass and irradiance.

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

“…Indeed, during the few hours of low tide, the variability can be so large that a few minutes between the measurements of NDVI (directly at the sediment surface) and those of CHLa (summed for the top layer of the sediment of 2 mm) might be too much for a direct comparison. A high temporal variability in NDVI was also reported in [77]. Thus, during the field campaign in September 2018, vertical migration of diatoms during the time lag between sampling and NDVI measurements might have caused that the conditions during the CHLa and NDVI measurements were not the same anymore.…”

The exploitation of biofilm by migrant western sandpipers (Calidris mauri)

“…Chlorophyll fluorescence [ 42 ] is widely used in botany and ecophysiological studies, including of marine and microalgae [ 4 ], and has previously been used to measure the Chl-a concentration of biofilm in both laboratory (in surficial sediments transported to the lab [ 61 ]); and field [ 33 , 50 ] studies. We used a field-portable chlorofluorometer (Opti-science model CCM-300) to make Chl- a density measurements on Roberts Bank during western sandpiper northward migration during two years.…”

“…western sandpipers ( Calidris mauri )) ‘graze’ intertidal biofilm [ 18 , 34 , 35 , 41 ], a thin layer (up to a few millimeters thick) at the sediment-water interface consisting of microorganisms, detritus, and sediment [ 2 , 65 , 70 ] suspended in a matrix of polymers. Biofilms dominate the primary production of turbid, temperate estuaries [ 33 ] and are a food source for many invertebrates [ 24 ]. Sandpipers graze biofilm using rapid pecking-like motions and a modified tongue which they dab onto the mudflat surface.…”

A review of carbon monitoring in wet carbon systems using remote sensing

“…Temporal dynamics of spectral indicators of biomass, i.e. normalized difference vegetation index, were explored in tidal flats, too [142,143]. Most tidal system studies (n = 33) pertained to marshes dictating this section's focus.…”