Abrupt shifts in stream algal biomass and diatom taxonomic composition along a gradient of changing land use

Variation in watershed land use is known to influence the physical and chemical conditions in resident streams; however, few studies have evaluated variation in ecosystem structure and function, in a quantitative sense, whereby specific environmental break-points in land use could be defined. We conducted seasonal sampling (spring, summer, and fall of 2005-2006) in 43 third order streams that were representative of conditions throughout the mid-Atlantic region. Stream physical, biological, and chemical conditions were measured as represented by 19 environmental variables (physical-chemical), in addition to benthic chlorophyll concentrations and the relative abundance of diatom species collected from each stream. Watershed forest cover explained significant variation in stream benthic chlorophyll (stepwise linear regression, r² = 39.2%) as selected from a set of relevant variables (total nitrogen, total phosphorus, % Forest cover, and stream temperature). Benthic chlorophyll (range 0.1 to 917mg m^-2) and nutrient tolerant diatom guilds were inversely correlated with forest cover, while nutrient sensitive diatom species had a positive correlation. Loess trend analysis coupled with regression tree analysis (RPART) identified abrupt changes in benthic chlorophyll and the relative abundance of diatom nutrient guilds at breakpoints of 60% and 80% forest cover. Despite the correlative nature of this study, the findings here suggest individual ecosystems may exhibit abrupt changes in ecosystem function following declines in forest cover, particularly at the breakpoints identified here.