Dissolved organic material as a resource subsidy for phytoplankton in lake ecosystems

Background/Question/Methods

Dissolved organic material (DOM) is one of the most important regulators of plankton ecology in lakes. It strongly controls vertical habitat gradients, including transparency to ultraviolet radiation as well as depth of the mixed layer. While the role of DOM as a regulator of physical gradients is well recognized, the role of this material as a resource subsidy to plankton remains unclear. DOM includes various forms of bioavailable nitrogen (N) and phosphorus (P), which are frequently limiting nutrients in lake ecosystems. With increases in DOM concentrations observed in lakes in most regions of the northern hemisphere, this raises numerous questions about the implications for plankton ecology through the dual roles of this material. We focused on investigating potential resource subsidy effects of DOM. Microcosm experiments were conducted with natural phytoplankton assemblages to assess their growth and community structure in two different treatments: DOM enrichment or DOM shading. Experiments were conducted across three lakes in the northern US: one in an alpine lake in the Rocky Mountains, and two in boreal lakes of Michigan and Maine. To simulate realistic changes occurring in these lakes, DOM additions were 1 mg L^-1 and were extracted from local vegetation on each watershed.

 Results/Conclusions

The three lakes in this study exhibited three different nutrient limitation patterns, with P-limitation in the alpine lake and either N-limitation or N and P co-limitation in the boreal lakes. Regardless of nutrient limitation pattern, algal biomass increased with DOM enrichment in all three lakes, suggesting the DOM was adding both N and P that stimulated phytoplankton growth. When DOM was only used to shade algae (i.e., not added directly to microcosms but layered on top of them), no differences were observed compared to the control, suggesting that light attenuation by DOM was not the mechanism by which DOM enrichment stimulated algal growth. Autotrophic algae comprised the increased algal biomass in the DOM additions, with diatoms increasing in the alpine lake, and chlorophyte taxa increasing in the boreal lakes. Our results suggest that DOM may serve as an important resource for phytoplankton, and this role should be more tightly integrated into our understanding of the links between increasing DOM and changing plankton dynamics in lake ecosystems.