Using opportunistic green macroalgae as indicators of nitrogen supply and sources to estuaries

Nutrient inputs to estuaries are increasing worldwide, and anthropogenic contributions are increasingly complex and difficult to distinguish. Measurement of integrated effects of salinity and nutrient changes simultaneously can help ascertain whether N sources of similar magnitude and stable isotope (δ¹⁵N) signatures are river dominated. We used Enteromorpha spp., an opportunistic macroalga, to obtain integrated measures of salinity, nutrient supply, and nutrient source in estuaries. We outplanted cultured algae in the field along spatial gradients within three southern California estuaries for 24 hours in wet and dry seasons. Tissue was analyzed for potassium (K ⁺) to measure osmoregulatory changes, total nitrogen to examine changes in nutrient supply, and δ¹⁵N to assess nutrient sources. Discrete measures of water salinity correlated with tissue K ⁺ content; however, there was significant variability in the relationship, suggesting that the algae were subject to considerable variation in salinity over a tidal cycle. Tissue total N was not always related to snapshot measures of water column dissolved inorganic nitrogen (DIN), suggesting that integrated measures may be better at capturing the temporal and spatial complexity of nutrient availability. The combination of tissue K⁺, total N, and δ¹⁵N measures revealed that inflowing rivers delivered N from watershed sources to Mugu Lagoon and Carpinteria Salt Marsh, while both the inflowing river and a mid estuary source were important sources of high δ¹⁵N N in Upper Newport Bay. These experiments revealed complex patterns of supply and sources of N and demonstrate the usefulness of macroalgal indicators over water sampling to detect these patterns.