Influences of dissolved organic matter on phytoplankton functional diversity and resource use efficiency differ in three lakes with contrasting trophic state and depth

Dissolved organic matter (DOM) plays a crucial role in both the penetration of light underwater and the biogeochemical cycles of carbon and nutrients in lakes, thereby largely influencing phytoplankton community structure and ecosystem functioning. However, how DOM affects phytoplankton functional diversity (FD) and resource use efficiency (RUE), especially in contrasting lake types is not well-known. In the study reported here, we explored the influences of DOM on phytoplankton FD and the RUEs of nitrogen and phosphorus in three lakes (i.e., a deep oligotrophic lake, Lake Fuxian; a shallow mesotrophic lake, Lake Erhai, and a shallow eutrophic lake, Lake Qilu), all located in southwest China. We found that the relationships between DOM properties and phytoplankton composition, FD and RUEs differed in these three lakes and exhibited seasonality. Partial least squares path modeling revealed the different pathways by which dissolved organic carbon (DOC) concentration influenced phytoplankton FD and RUE between the lakes. In Lake Fuxian, DOC promoted RUE by positively affecting phytoplankton FD and indirectly regulated FD and RUE by affecting water transparency and temperature. In Lake Erhai, DOC affected RUE by altering nutrient supplements. In Lake Qilu, in addition to the negative direct influence of DOC on FD, DOC affected FD or RUE by regulating water transparency and temperature. These results imply that the modulation of DOM on available resources will shift across lake depth and trophic state gradients, profoundly affecting phytoplankton diversity and/or ecosystem functioning. Our study provides a reference for furthering current understanding of biodiversity‒ecosystem functioning in lakes, particularly within the context of environmental pressures such as increased DOM and nutrient levels.