Effects of experimental manipulation of algae and fauna on the properties of intertidal soft sediments

Amounts of algae and fauna were manipulated on an intertidal mudflat to test the hypothesis that removal of fauna would affect properties of the sediment in different ways than would removal of algae. Pesticide and algicide were used to selectively remove fauna and micro-algae from the sediment, respectively. Experimental plots were either sprayed with pesticide, algicide or freshwater (as a test for the effects of spraying). Controls were not sprayed. Treatments were reapplied on days 10 and 16 and all data were collected on day 23 of the experiment. Sediment samples were taken for laboratory determination of sediment grain-size and concentration of water, colloidal and total carbohydrate, chlorophyll-a and organic matter. Reflectance spectra and minimal fluorescence were acquired to provide information on the types of plant pigments present in each treatment and their abundance. Pesticide significantly reduced numbers some groups of animals (e.g. Nereids) but not others (e.g. Capitellids and Nematodes). Removal of fauna did not significantly affect any sediment property, with the exception of colloidal carbohydrate which showed an increase. This suggests a degree of functional redundancy in the system or idiosyncratic effects, where the function of animals removed by the pesticide was replaced by the animals that remained. Removal of algae had a profound effect on numbers of animals (e.g. reduced numbers of Nereids, Capitellids, Oligochaetes, Nematodes, Salinator, Ophistobranch molluscs and Sabellids) and on the properties of the sediment. The amount of sediment grains greater than and less than 63 µm increased and decreased, respectively as a result of the reduction in amounts of algae. Colloidal and total carbohydrate, organic matter, water and chlorophyll all decreased in the algicide treatment in line with predictions. This experiment underscores the need to take into account both direct and indirect effects of treatments in manipulative experiments.