Temporal and spatial heterogeneity in lacustrine δ13CDIC and δ18ODO signatures in a large mid-latitude temperate lake

Modelling limnetic carbon processes is necessary for accurate global carbon models and stable isotope analysis can provide additional insight of carbon flow pathways. This research examined the spatial and temporal complexity of carbon cycling in a large temperate lake. Dissolved inorganic carbon (DIC) is utilised by photosynthetic organisms and dissolved oxygen (DO) is used by heterotrophic organisms during respiration. Thus the spatial heterogeneity in the pelagic metabolic balance in Loch Lomond, Scotland was investigated using a combined natural abundance isotope technique. The isotopic signatures of dissolved inorganic carbon (delta C-13(DIC)) and dissolved oxygen (delta O-18(DO)) were measured concurrently on four different dates between November 2004 and September 2005. We measured isotopic variation over small and large spatial scales, both horizontal distance and depth. delta C-13(DIC) and delta O-18(DO) changed over a seasonal cycle, becoming concurrently more positive (negative) in the summer (winter) months, responding to increased photosynthetic and respiratory rates, respectively. With increasing depth, delta C-13(DIC) became more negative and delta O-18(DO) more positive, reflecting the shift to a respiration-dominated system. The horizontal distribution of delta C-13(DIC) and delta O-18(DO) in the epilimnion was heterogeneous. In general, the south basin had the most positive delta C-13(DIC), becoming more negative with increasing latitude, except in winter when the opposite pattern was observed. Areas of local variation were often observed near inflows. Clearly delta C-13(DIC) and delta O-18(DO) can show large spatial heterogeneity, as a result of varying metabolic balance coupled with inflow proximity and thus single point sampling to extrapolate whole lake metabolic patterns can result in error when modelling large lake systems Whilst we advise caution when using single point representation, we also show that this combined isotopic approach has potential to assist in constructing detailed lake carbon models.