Abstract. Stable isotopes (particularly C and N) are widely used to make inferences regarding food web structure and the phenology of consumer diet shifts, applications that require accurate isotopic characterization of trophic resources to avoid biased inferences of feeding relationships. For example, most isotope mixing models require that endmembers be adequately represented by a single probability distribution; yet, there is mounting evidence that the isotopic composition of aquatic organisms often used as mixing model endmembers can change over periods of weeks to months. A review of the literature indicated that the d13 C values of five aquatic primary consumer taxa, commonly used as proxies of carbon production sources (i.e., trophic baselines), express seasonally dynamic cycles characterized by an oscillation between summer maxima and winter minima. Based on these results, we built a dynamic baseline mixing model that allows a growing consumer to track temporal gradients in the isotopic baselines of a food web. Simulations showed that the ability of a consumer to maintain or approach isotopic equilibrium with its diet over a realistic growth season was strongly affected by both the rate of change of the isotopic baseline and equilibration rate of the consumer. In an empirical application, mixing models of varying complexity were used to estimate the relative contribution of benthic vs. pelagic carbon sources to nine species of juvenile fish in a fluvial lake of the St. Lawrence River system (Que´bec, Canada). Estimates of p (proportion of carbon derived from benthic sources) derived from a static mixing model indicated broad interspecific variation in trophic niche, ranging from complete benthivory to .95% reliance on pelagic food webs. Output from the more realistic dynamic baseline mixing model increased estimated benthivory by an average of 36% among species. Taken together, our results demonstrate that failing to identify dynamic baselines when present, and (or) matching consumers with baseline taxa that possess substantially different equilibration rates can seriously bias interpretation of stable isotope data. Additionally, by providing a formalized framework that allows both resources and consumers to shift their isotopic value through time, our model demonstrates a feasible approach for incorporating temporally dynamic isotope conditions in trophic studies of higher consumers.

Increased nutrient loads stimulate estuary primary productivity and can alter the structure and function of biological communities within estuaries, particularly when producer groups respond differently to changes in nutrient availability. Here, the relative influence of riverine inputs of nitrogen and phosphorus were compared to determine their contribution to estuarine primary producers at large spatial scales. Indices of demersal (extent of macroalgae relative to other vegetation, total shallow water area vegetated) and planktonic (seasonally averaged chlorophyll concentration) primary producer communities were derived at whole-of-ecosystem scales in 14 estuaries dispersed across a longitudinal gradient using aerial imaging, underwater videography and in situ monitoring. A model selection framework was used to relate annual nutrient loads (total nitrogen [TN], dissolved inorganic nitrogen [DIN], total phosphorus [TP]), sediment loads (TSS), molar stoichiometric load ratios (TN M : TP M ), and estuary water residence times to the demersal and planktonic indices. Dissolved inorganic nitrogen was the best predictor of the extent of macroalgae, total vegetation coverage, and the concentration of planktonic chlorophyll. Rapid increases in all three indices occurred at inorganic nitrogen loads of 5-10 Mg km 22 yr 21 . There was some evidence that TN M : TP M , TP and TSS loads were informative model covariates. Relative to DIN loads, TP loads were a poor predictor of the macroalgal and planktonic indices. These findings underscore the critical role of catchment-derived nitrogen in contributing to producer communities at the whole-of-ecosystem scale and support the growing consensus that nitrogen loads (in addition to phosphorus) must be managed to effectively alleviate eutrophication in estuaries.

Seasonal oscillations in the carbon (δ(13)C) and nitrogen (δ(15)N) isotope signatures of aquatic algae can cause seasonal enrichment-depletion cycles in the isotopic composition of planktonic invertebrates (e.g., copepods). Yet, there is growing evidence that seasonal enrichment-depletion cycles also occur in the isotope signatures of larger invertebrate consumers, taxa used to define reference points in isotope-based trophic models (e.g., trophic baselines). To evaluate the general assumption of temporal stability in non-zooplankton aquatic invertebrates, δ(13)C and δ(15)N time series data from the literature were analyzed for seasonality and the influence of biotic (feeding group) and abiotic (trophic state, climate regime) factors on isotope temporal patterns. The amplitude of δ(13)C and δ(15)N enrichment-depletion cycles was negatively related to body size, although all size-classes of invertebrates displayed a winter-to-summer enrichment in δ(13)C and depletion in δ(15)N. Among feeding groups, periphytic grazers were more variable and displayed larger temporal changes in δ(13)C than detritivores. For nitrogen, temporal variability and magnitude of directional change of δ(15)N was most strongly related to ecosystem trophic state (eutrophic > mesotrophic, oligotrophic). This study provides evidence of seasonality in the isotopic composition of aquatic invertebrates across very broad geographical and ecological gradients as well as identifying factors that are likely to modulate the strength and variability of seasonality. These results emphasize the need for researchers to recognize the likelihood of temporal changes in non-zooplankton aquatic invertebrate consumers at time scales relevant to seasonal studies and, if present, to account for temporal dynamics in isotope trophic models.