CDOM and the underwater light climate in two shallow North Patagonian lakes: evaluating the effects on nano and microphytoplankton community structure

Abstract

We performed an annual synchronous sampling in two oligotrophic shallow lakes to assess the influence of chromophoric dissolved organic matter (CDOM) on the underwater light climate, and its potential effects on the nano and microphytoplankton community structure. Lake Escondido showed higher CDOM concentration and light attenuation with a spectral composition of underwater light shifted towards green–yellow light, while Lake Morenito presented clearer waters and a dominance of green light. Temporal dynamics of CDOM absorption at 440 nm were consistently explained by differences in cumulative precipitation. Mixotrophic cryptophytes and chrysophytes dominated the phytoplankton of both lakes, although the prevalence of each algal group was different between lakes. The dominance of these groups was largely explained by differences in spectral composition of underwater light, estimated as the ratio between Kd(RED) and Kd(GREEN) [Kd(R)/Kd(G) ratio]. Cryptophytes prevailed in Lake Morenito and their biomass showed a positive strong relationship with Kd(R)/Kd(G) ratio. Chrysophyte biomass was comparatively more important in Lake Escondido showing an opposite relationship with the Kd(R)/Kd(G) ratio. These results underscore that higher relative green light availability allowed the dominance of cryptophytes, while changes in light spectral composition driven by CDOM allowed coexistence. We suggest that nano and microphytoplankton community structure in these lakes could be driven by changes in spectral composition of underwater light shaped by differences in CDOM, ultimately determined by precipitation/hydrological patterns.