Spatial and temporal variation in sediment properties of an intertidal mangrove forest: implications for sampling

Studies of the structure or functioning of intertidal soft sediments often involves collection of biogeochemical data over tidal, diel and seasonal time-scales. Little effort has, however, been made to quantify accurately the time-scales at which these properties vary. Many previous studies collected samples from different sites at different times, potentially resulting in the confounding of spatial and temporal variation. This experiment was designed to determine if time of day or time within the exposure period had any significant effect on measurements of 7 different properties of sediments. Samples of sediment were collected using contact cores at the beginning, middle and end of tidal emersion at each of 9 a.m., 12 p.m. and 3 p.m., with two replicate days of each condition, from mangrove forests fringing Glades Bay, Sydney, Australia. These samples were analysed for water-content, pigments, carbohydrates, grain-size and loss on ignition (LOI), to determine the potential effects of time of day and time within the tidal cycle on these properties of the sediments.

Whilst both time of day and time within the tidal cycle were found to occasionally have a significant effect upon the measured properties, most of the variation occurred among sites and between replicate days of each set of conditions. The minimal influence of time of day and time within the tidal cycle show that sampling effort should be preferentially placed into replicating days of sampling and sites because these are the scales with the greatest variation. Differences in the patterns found also depended on whether the data were expressed as content or concentration, the consequences of which are briefly described.