We investigated nutrient limitation of periphyton and phytoplankton in Cape Cod, MA coastal plain freshwater ponds. We assayed periphyton growth response to nitrogen (N) and phosphorus (P) in situ, assessed phytoplankton growth in growth chambers, and measured ratios of dissolved N and P in surface waters to determine if nutrient ratios were accurate predictors of nutrient limitation. In ponds receiving low groundwater nutrient inputs, periphyton responded weakly to addition of N or P alone, but responded strongly to addition of N P. In these ponds, increases in both N and P were also required to increase phytoplankton growth. In ponds receiving high groundwater nutrient inputs, increased N P were also required to increase phytoplankton growth. We found no indication that high nutrient inputs shifted pond phytoplankton to P limitation. There was no consistent correlation between dissolved N:P and whether periphyton was limited by either N or P or co-limited by N and P. Strong and widespread co-limitation by N and P suggested that greater attention should be given to both N and P when assessing the threat of nutrient enrichment to fresh waters. Effects of increased periphyton and phytoplankton growth on the unique flora of coastal plain ponds are not known, but bear increased attention given large increases of N in groundwater in many locations, the sensitivity of pond algae to nutrient supply, and the status of coastal plain ponds as high conservation priorities.
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