Changes in microphytobenthic chlorophyll a resulting from sediment compaction due to de-watering: opposing patterns in concentration and content

The study of estuarine microphytobenthos often involves the observation of data over diel, tidal and seasonal timescales. Algal biomass is usually measured using the proxy of chlorophyll a (Chl a) content or concentration. However, during tidal emersion, exposed sediments often experience desiccation, leading to de-watering and an increase in sediment wet bulk density. If these changes are ignored, incorrect patterns can be attributed to the data obtained, such as an apparent increase or decrease in biomass over the emersion period. This study describes predicted changes in Chl a calculated due to de-watering over a 6 h emersion period and compares this data to observed diel and seasonal variation in sediment Chl a. Predicted Chl a content (mug g(-1) dw sediment) significantly decreased due to de-watering by 13-16%, dependent upon the depth of sediment sampled. However, the same data expressed as Chl a concentration mug cm(-2)) showed a significant increase (40-47%) in biomass. Over a 14 h diel incubation, under ambient light without tidal immersion, actual Chl a concentration in sediment cores showed a linear increase of 86%, probably as a result of de-watering. This change may mask a possible endogenous rhythm observed for the same data expressed as Chl a content. In contrast, seasonal-Chl a content showed a significant positive correlation with percentage water content, while no pattern was observed for seasonal Chl a concentration. Extracellular polymeric substance (EPS), a water soluble carbohydrate, showed opposite patterns to Chl a over tidal emersion, with content showing a positive correlation and concentration a negative correlation with water content. This difference was attributed to EPS being a highly hydrated, soluble substance, whereas Chl a is independent of, the aqueous phase. It is shown that changes in water content must be incorporated into calculation of all sediment data where de-watering over time may occur. In addition, care should be taken when expressing data as a content or concentration. (C) 2003 Elsevier Science Ltd. All rights reserved.