Positive and negative effects of riverine input on the estuarine green alga Ulva intestinalis (syn. Enteromorpha intestinalis) (Linneaus)

Freshwater inputs from rivers alter salinity of estuaries, and are also important conduits for the delivery of nutrients such as nitrogen and phosphorus. We studied the impact of freshwater inputs on primary producers in the lower Housatonic River estuary in Long Island Sound, U.S.A. We conducted a laboratory experiment with Ulva intestinalis (syn. Enteromorpha intestinalis) (Linnaeus), a common green macroalgae that can have a high biomass in eutrophic systems. U. intestinalis was collected from three sites around the estuary that varied in salinity and nutrient concentration. Algae from three sites were grown in four treatments containing different proportions of Housatonic River water to mimic the gradient in riverine influence in the estuary. As the percentage of Housatonic River water increased, nitrogen and phosphorus concentration increased and salinity decreased. Growth of U. intestinalis collected from lower salinity sites was higher in treatments containing Housatonic River water than in those containing only Long Island Sound water. Conversely, U. intestinalis collected from Long Island Sound grew best in the treatment with no river water. Previous studies showed that U. intestinalis growth is stimulated by high nutrient concentration and depressed by low salinity; however, the reduction in growth at low salinity may be mitigated by increased nutrients. Our results support these studies and suggest that for populations of U. intestinalis that have experienced reduced salinity in their environment, the negative impacts of reduced salinity may be outweighed by the positive impacts of the high nutrient concentration in Housatonic River water.