High-resolution temporal NDVI data reveal contrasting intratidal, spring-neap and seasonal biomass dynamics in euglenoid- and diatom-dominated biofilms.

Intertidal microphytobenthos (MPB) are a major contributor to primary production in estuarine ecosystems. While their biomass is highly variable at multiple spatial and temporal scales, the underlying drivers are as yet little understood. Both in situ sampling and remote-sensing techniques often lack the temporal resolution or coverage to simultaneously capture short-term (intratidal to daily) and longer-term (weekly to annual) biomass changes. Our field setup with in-situ NDVI sensors allowed us to study MPB surface biomass variability at high temporal resolution (10 mins) for up to two years in a freshwater euglenoid dominated mudflat, and a brackish and a marine diatom dominated mudflat. MPB biomass showed marked periodicities at multiple temporal scales: seasonal, spring-neap and intratidal. The diatom-dominated MPB community showed a seasonal biomass peak in winter, while the euglenoid-dominated community showed biomass peaks during spring and summer, probably caused by underlying divergent responses to mainly irradiance, temperature and wind-induced resuspension, and macrobenthos grazing. Spring-neap periodicity likely resulted from differential migratory responses of the MPB communities to variation in timing and duration of daylight exposure. In the freshwater community, upward migration only occurred when exposure duration was sufficiently long (≥4 h). In the diatom-dominated community, morning daylight exposure resulted in highest NDVI values. This study highlights the differences in MPB biomass dynamics between MPB communities within estuarine ecosystems, and underscores the great potential of high-resolution temporal NDVI monitoring for more accurate estimates of MPB biomass and primary production.

Input, behaviour and distribution of multiple elements in abiotic matrices along a transect within the Okavango Delta, northern Botswana.

Wetlands fed by rivers can be a sink for elements depending on elemental concentrations, wetland hydrology, geochemistry, vegetation and climate. In the case of the Okavango Delta, northern Botswana, the outflow discharge is a small fraction (2-5%) of the inflow. This has strong potential consequences for the Delta, as it strongly affects element cycling and storage within the Delta. We estimated the inputs, behaviour and distribution of multiple elements along a longitudinal transect within the Okavango Delta, to show potential effects of retention mechanisms of different elements. High annual element input is rather attributed to discharge than to the concentration within the water, which is generally extremely low. We observed minimal enrichment of the elements within the water pathway along the transect from inflow to outlets, implying that element output is negligible. For most elements, we observed a high correlation between storage and sediment organic matter content. The organic matter content within the sediments was higher in the vegetated sediments than in non-vegetated sediments (factor âˆ¼ 10), and a similar trend was found for most elements. In conclusion, organic matter dominated in sediments from vegetated plots and thus plays an important role in retaining the elements within the sediments of the Delta. This finding has major implications for e.g. planning constructed wetlands for water purification or element retention especially in areas with high evapotranspiration.