The spatio-temporal patterns and ecological significance of free amino acids and humic substances in contrasting oligotrophic and eutrophic freshwater ecosystems

Abstract

The concentrations of total and individual free amino-acids (TFAA, FAA) and humic substances (HS) were measured by HPLC in the midwater column (MWC), air-water interface (AWI) and sediment pore water (SPW) in the oligotrophic, dystrophic Isle of Thorns lake and the eutrophic Lewes Brooks drainage channel in East Sussex, UK, during each of the four seasons. Pesticide levels were low, usually below detection levels in both water bodies. At the temporal level there were tendencies for the concentration of both FAA and HS to be lowest in the winter months and for the HS concentrations to be higher in the autumn in both water bodies. At the spatial level the eutrophic Lewes Brooks was characterized by having much higher FAA/HS ratios than the oligotrophic Isle of Thorns lake and it is suggested that further comparative studies be undertaken to verify whether the FAA/HS ratios may have general application in the classification of water bodies on a trophic basis. The concentrations of TFAA are higher in the SPW and AWI than in the MWC in both water bodies and the HS concentrations are much higher in the SPW than in the MCW Statistical analyses support the view that the FAA in the MWC are mainly derived from the SPW whereas the FAA in the AWI have a different origin. The data obtained is used to address the following questions: (a) How do the spatio-temporal patterns of FAA and HS in the present study areas compare with those of other water bodies? (b) What are the origins of the FAA and HS and how are their concentrations regulated? (c) Are the distributional patterns of FAA and HS of special significance to invertebrate detritivores? (d) How can the differences in the FAA and HS patterns in oligotrophic and eutrophic waters be explained? What are the conclusions and testable hypotheses arising from the present study?