Patterns in the Concentration and Transport of Nutrients and Major Elements in the Watersheds and Embayments of Lake Ontario: a Landscape Perspective

Lake Ontario embayments were studied in a National Science Foundation (NSF) Biocomplexity Project to understand factors controlling the structure and function of freshwater ecosystems associated with Lake Ontario. Eight embayments of Lake Ontario and associated watersheds were selected to examine how the linkages between upland watersheds and Lake Ontario control the inputs and function of interconnected open freshwater embayments ecosystems. In this study, water samples were collected monthly from the eight Lake Ontario embayments, and 22 tributary sites in the associated sub-watersheds and Lake Ontario. Supporting hydrologic measurements were also made over the period 2001-2003. These data were used to examine water quality patterns in the upland, tributaries and embayments. Samples were analyzed for nutrients and major solutes. The land cover of the sub-watersheds was varied, but largely a mixture of forest and agricultural lands. Tributaries draining largely agricultural lands exhibited distinct seasonal patterns, particularly for nutrients. Nitrate and total nitrogen (TN) concentrations were generally low during the summer growing season; concentrations increased markedly during fall and decreased during winter and spring. Total phosphorus (TP), dissolved organic carbon (DOC) and major ion (Na+, K+, Ca2+, Mg2+, Cl-, SO42-) concentrations varied with seasonal discharge patterns. Distinct spatial patterns in tributary solute concentrations were observed that closely corresponded with land use. Solute concentrations increased markedly with increases in the percentage of the watershed occurring as agricultural land. Such a pattern has been commonly observed for nutrients (e.g., TP, TN, NO3- , DOC), but this relationship was also strongly evident for many non-nutrient solutes (e.g., F-, SO42-). These observations suggest that agricultural activities mobilize most elements and greatly enhance transport across the temperate landscape impacting surface water resources. The coastal embayments of Lake Ontario obtain their water from upland watershed drainage, direct precipitation and mixing with Lake Ontario. A 3-end member mixing analysis was applied using fluoride as the conservative solute to study the mixing patterns for the embayment ecosystems. Results indicated that more than 60% of waters were derived from the adjacent watershed in most embayments annually. However, more than 90% of water in Little Sodus was derived from Lake Ontario. Direct precipitation contributed about 10% in some embayments with large surface area. Water residence times were calculated in a range between less than 1 day in Floodwood to 148 days for South Sandy Pond using the 3-end member mixing model. Seasonal variations in the source of water to Lake Ontario embayments were observed, with watershed discharge dominating in spring and Lake Ontario inputs increasing during the low flow summer period. Significant contributions from direct precipitation occurred in late summer and early fall, coinciding with low watershed inputs. The embayment ecosystems receive nutrient inputs from upland watersheds and water exchange with Lake Ontario, and undergo biogeochemical processes through internal nutrient cycles simultaneously. Regression analyses suggested that upland watersheds played important roles in TN and DOC dynamics in the study embayments. A positive correlation between TP and density of residences adjacent to the embayments may reflect the release of TP from lawn or garden fertilizers and septic tanks contributing to TP input to embayments. Mass balance calculations suggested that internal processes within embayments play important roles for some nutrient dynamics. For example, study embayments acted as sinks for dissolved silica and nitrate, but a source for ammonium. Watershed runoff and inflow from Lake Ontario transported acid neutralizing capacity (ANC) into embayment ecosystems. Mass balance models for dissolved inorganic carbon, ANC and particulate carbon were developed to improve understanding of the carbon cycle in the linked freshwater ecosystems. Regression analysis indicated that watersheds with considerable agricultural land use were large sources of ANC in stream discharge, and internal biogeochemical processes contributed to changes in ANC in the embayments with longer water residence times. Both mineral saturation index and mass balance calculations indicated the potential for CaCO3 precipitation, suggesting that embayment ecosystems could be terrestrial sinks of inorganic carbon. Also, retention of SO42- and NO3- can contribute to production of ANC within embayments