Macrophyte assemblages in ditches of coastal marshes in relation to land-use, salinity and water quality

Historical land claim of coastal salt marshes in eastern England resulted in the development of grazing marshes, many of which were subsequently converted to arable farming. Their ditches support a rich variety of macrophyte species, due to subtle changes in water chemistry and hydroperiod. Macrophytes were surveyed in 36 shallow freshwater/brackish marsh ditches in eastern England during spring/summer 2000 to determine the relationships between environmental factors, agricultural activities (grazing or arable land use), and macrophyte diversity. Pasture site ditches supported a higher number of species (21) than arable ditches (8). Principle Components Analysis (PCA) for environmental variables associated pasture ditches with higher sediment ash-free dry weight (AFDW) and lower orthophosphate concentration and salinity. TWINSPAN classification grouped the submerged species Callitriche stagnalis, Ceratophyllum demersum, Ranunculus peltatus, R. aquatilis. Myriophyllum spica tum, and Potamogeton crispus into pasture ditch sites while the emergent species Phragmites australis and Scirpus maritimus were dominant in arable ditches. Canonical Correspondence Analysis (CCA) for all environmental variables analyzed showed that macrophyte distribution was strongly related to salinity, dissolved inorganic nitrogen (DIN) and sediment AFDW, which differed between pasture and arable sites. Increases in salinity and decreases in AFDW were associated with decreasing species richness (e.g. submerged and floating species). The growth of two submerged macrophytes (C. demersum and C. stagnalis) was studied in transplant experiments over three months in ditches at opposite ends of the TWINSPAN classification. In pasture ditches both species showed significant net biomass increment, whereas in arable ditches plants had significantly lower growth rates, with reduced biomass in some ditches. Macrophyte net biomass was predicted by a combination of salinity, water depth and water column Chl-a. Experimentally, salinity above 3 ppt significantly reduced the net growth of C. stagnalis, although C. demer sum grew in salinities up to 5 ppt, above which leaf damage and necrosis occurred. Salinity up to 6 ppt was detected in arable ditches and was highly negatively correlated with water depth. Results suggest that ditches flora is affected in various ways based on surrounding land use. Salinity control by increased freshwater input and reduction of nutrient load are important factors in maintaining diversity of macrophyte assemblages. © 2008 E. Schweizerbart'sche Verlagsbuchhandlung.