Impact of tributary DOM and nutrient inputs on the nearshore ecology of a large, oligotrophic lake (Georgian Bay, Lake Huron, Canada)
2013
Bocaniov, Serghei A. | Barton, David R. | Schiff, Sherry L. | Smith, Ralph E. H.
Two adjacent bays in a large oligotrophic lake (Georgian Bay, Lake Huron) were compared to determine how the inputs from relatively pristine, but moderately humic, tributaries may influence phytoplankton, nutrients and system metabolism. Dissolved organic carbon (DOC) concentrations decreased from 4 to 5 gC m⁻³ at inner sites to 2 gC m⁻³ or less at outer sites. The concentration gradient from inner to outer was greater in the bay with a major tributary, and optical properties (intensity and slope of light absorption spectrum) showed there was a loss of material with allochthonous characteristics along the gradient. Chlorophyll a (Chl a) and total phosphorus (TP) were also higher at inner (2–4 mg Chl a m⁻³ and 8–12 mgP m⁻³, respectively) than outer sites (≤1 mg Chl a m⁻³ and 4–5 mgP m⁻³). Chl a and TP, as well as particulate nutrient ratios (C:P, C:N, N:P), indicated significant eutrophication at inner sites, especially in the bay with the tributary, and there was a strong positive Chl a-phosphorus relationship. The stable oxygen isotope ratio (¹⁸O:¹⁶O) of dissolved oxygen indicated greater influence of biological oxygen fluxes at inner sites (where ratios were 2–3 ppt below atmospheric equilibrium) than at outer sites (where ratios were within 0.5 ppt of equilibrium). Community photosynthesis:respiration ratios inferred from ¹⁸O:¹⁶O varied positively with Chl a and inorganic nutrients, but negatively with DOC. Altered loading of allochthonous organic matter can be expected under changing climate and development scenarios and will have significant influence on optical properties and system metabolism through changes in DOC in this coastal system. The effects will nonetheless be strongly modulated by any accompanying change in inorganic nutrients.
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