Periphyton response to nutrient and light limitation in a tropical urban river, Puerto Rico

Puerto Rico is a highly urbanized tropical island with 16% of its surface under urban land use. Most urbanization is concentrated in the San Juan metropolitan area (population density: 3,500 people/km2). We have limited information about how urban development impacts river ecosystems on the island. Therefore, the objectives of this study were to: 1) identify whether urban rivers are impacted by nutrient contamination, 2) determine which nutrient (e.g., phosphorus or nitrogen) is most limiting, and 3) assess the response of  periphyton to contrasting nutrient and light levels. We selected the Río Piedras, in San Juan, as it is impacted by urban runoff and waste water from leaking pipes. Two reaches with contrasting canopy covers were chosen to assess light limitation on periphyton. Periphyton was characterized as chlorophyll-a and ash-free dry mass (AFDM) and nutrient limitation was assessed using nutrient diffusing substrates and alkaline phosphatase activity.  We found high chlorophyll-a and AFDM levels, relative to non-urban rivers in Puerto Rico.  Measures of nutrient limitation indicated phosphorus limitation, potentially due to high nitrate levels in the river. Overall, the Río Piedras is severely impacted by urbanization and nutrient levels are impacting algal communities and increasing their biomass relative to non-urban rivers.