Hypolimnetic Nutrient Subsidies to Surface Waters of a Large Lake: A Coupled Hydrodynamic Modelling and Nitrogen Isotope Field Assessment

Abstract

Upwelling of hypolimnetic water in lakes is important for littoral food webs as it enhances production through nutrient subsidies to surface waters. The aim of this study was to understand the role of hypolimnetic upwelling on littoral nutrient supply and diet of a freshwater unionid mussel, kākahi (Echyridella menziesii) in an oligotrophic lake using stable isotopes (δ15N) as a tracer of hypolimnetic nitrogen. Upwelling events can be highly variable spatially and temporally, so to complement observations, this study used a three-dimensional hydrodynamic model (AEM3D) to simulate the transport of water from upwelling events and understand their potential impacts on kākahi. The study was conducted over an annual cycle in a large (area 620 km2, average depth ~ 94 m), monomictic, oligotrophic New Zealand lake. The hydrodynamic model simulations showed a strong spatial gradient. Upwelling was strongest along north-facing areas of the western shoreline owing to the prevailing south-westerly winds. The spatio-temporal distribution of nitrate-δ15N within the littoral zone was positively related to the rate of simulated upwelling, indicating inputs of 15N-enriched hypolimnetic water. Kākahi δ15N values were positively related to upwelling averaged over an annual period, demonstrating that upwelling hotspots are ecologically meaningful. Interestingly, littoral particulate organic matter δ15N showed no pattern with upwelling but were homogeneous in distribution, consistent with rapid flushing with offshore water. These findings suggest that benthic littoral habitats act as hotspots for nutrient subsidies to surface waters, and that the signature of 15N is retained over periods far longer than the duration of the upwelling by cycling within benthic primary producers and consumers.