Dissolved organic carbon concentration controls benthic primary production: Results from in situ chambers in north-temperate lakes

We evaluated several potential drivers of primary production by benthic algae (periphyton) in north-temperate lakes. We used continuous dissolved oxygen measurements from in situ benthic chambers to quantify primary production by periphyton at multiple depths across 11 lakes encompassing a broad range of dissolved organic carbon (DOC) and total phosphorous (TP) concentrations. Light-use efficiency (primary production per unit incident light) was inversely related to average light availability (% of surface light) in 7 of the 11 study lakes, indicating that benthic algal assemblages exhibit photoadaptation, likely through physiological or compositional changes. DOC alone explained 86% of the variability in log-transformed whole-lake benthic production rates. TP was not an important driver of benthic production via its effects on nutrient and light availability. This result is contrary to studies in other systems, but may be common in relatively pristine north-temperate lakes. Our simple empirical model may allow for the prediction of whole-lake benthic primary production from easily obtained measurements of DOC concentration. Ecosystem primary production plays an important role in governing the production of higher trophic levels (Lindeman 1942; Blanchard et al. 2012). Lakes are often used as discrete ecosystems to provide insights into the dynamics of ecosystem production (e.g., Carpenter et al. 1985). Though lake primary production was historically viewed as mostly phytoplankton-based, benthic algae (periphyton) make sizeable contributions to whole-lake primary production, particularly in shallow and clearwater lakes where periphyton dominate whole-lake production (Vadeboncoeur et al. 2001, 2003; Ask et al. 2009b). Stable isotope analyses have shown that benthic energy sources are also crucial to benthic invertebrates and fish (Hecky and Hesslein 1995). Despite the importance of periphyton to whole-lake production and the reliance of higher trophic levels on benthic production, it is only recently that benthic metabolism has been considered in lake ecosystem models (Vadeboncoeur et al. 2008; Jones et al. 2012; Jäger and Diehl 2014). Controls of benthic primary production are not universal across lakes. Where nutrient availability is naturally high or elevated due to anthropogenic inputs, phytoplankton biomass can regulate light availability and benthic primary production (Hansson 1992; Vadeboncoeur et al. 2003). Conversely, in cold, low-nutrient lakes where terrestrial organic matter often governs light availability, benthic production is driven by the concentration of chromophoric dissolved organic matter, which is often strongly correlated with, and measured as, dissolved organic carbon (DOC) concentration (Ask et al. 2009a). In the world’s largest lakes, nutrient status may determine benthic production in nearshore waters, while steep bathymetry reduces the relative contribution of periphyton to whole-lake production further offshore (Althouse et al. 2014). But these descriptions only represent the extreme ends of a lake type continuum and the controls of benthic production in intermediate lake types are unknown. Pristine north-temperate lakes constitute an ideal set of intermediate lake types because they encompass a wide range of DOC concentrations, have intermediate production, and experience cold winter and warm summer temperatures. We use the term ‘pristine’ here to describe lakes without significant human modifications of shoreline or the watershed. Recent conceptual syntheses describe how light and nutrient availability regulate benthic contributions to whole-lake primary production. Vadeboncoeur et al. (2008) modeled the proportional contribution of benthic algae to whole-lake primary production across a wide range of lake types and established that maximum periphyton contribution was determined by light availability at depth and the ratio of mean depth to maximum depth. Expanding on these concepts, Jones et al. (2012) developed a model that quantified the potential reduction in periphyton production as dissolved organic carbon inputs increased. Recently, Jäger and Diehl (2014) modeled the competitive interactions between benthic and pelagic primary producers across habitat boundaries, and validated their model using whole-lake production data from 27 lakes. These conceptual advances highlight the mechanistic processes crucial to primary production in lakes and demonstrate the importance of benthic production in these systems. However, they do not allow for straightforward estimations of benthic production for individual lakes, nor do they provide empirical evidence for the effects of light attenuation determinants on benthic production. * Corresponding author: [email protected] Limnol. Oceanogr., 59(6), 2014, 2112–2120 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.6.2112