Warmer Winters Increase the Biomass of Phytoplankton in a Large Floodplain River. Issue 9 (14th September 2021)

• Record Type: Journal Article
• Authors: Jankowski, Kathi Jo; Houser, Jeffrey N.; Scheuerell, Mark D.; Smits, Adrianne P.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 9(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-09-14
• Subjects: Mississippi River -- winter -- phytoplankton -- chlorophyll -- MARSS -- synchrony; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2020JG006135
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Winters are changing rapidly across the globe but the implications for aquatic productivity and food webs are not well understood. In addition, the degree to which winter dynamics in aquatic systems respond to large‐scale climate versus ecosystem‐level factors is unclear but important for understanding and managing potential changes. We used a unique winter data set from the Upper Mississippi River System to explore spatial and temporal patterns in phytoplankton biomass (chlorophyll a, CHL) and associated environmental covariates across 25 years and ∼1, 500 river km. To assess the role of regional climate versus site‐specific drivers of winter CHL, we evaluated whether there were coherent long‐term CHL dynamics from north to south and across lotic‐lentic areas. We then estimated the degree to which these patterns were associated with climate variability (i.e., the Multivariate El Nino‐Southern Oscillation Index), winter severity (freezing degree days), river discharge, or site‐specific environmental variables (ice depth, snow depth, and nutrient concentrations). We found that winter CHL was typically highest in ice‐free reaches and backwater lakes, occasionally exceeding summer values. We did not find highly synchronous CHL dynamics across the basin, but instead show that temporal trends were independent among river reaches and lotic‐lentic areas of the river. Moreover, after accounting for these spatial dynamics, we found that CHL was most responsive to winter air temperature, being consistently higher in years with warmer winters across the basin. These results indicate that although productivity dynamics are highly dynamic within large river ecosystems, changes in the duration and severity of winter may uniformly increase wintertime productivity. Plain Language Summary: Winters are getting shorter and warmer across the globe, but how this affects rivers is not well known. This is particularly true for large rivers, such as the Mississippi River, that provide important ecological and social services at continental scales. Here, we used 25 years of data collected during the middle of winter from the Upper Mississippi River to understand long term changes in the abundance of algae, an important food resource for fish and other river organisms. We found that the quantity of algae during the winter was comparable to summer in some years and areas of the river. In addition, we found that warmer winters had more algae, which could have implications for food webs as winters change. Finally, we show that the diversity of habitats within this large river ecosystem (e.g., lakes and flowing channels) had distinct trends in algal abundance over time, providing options for consumers and helping to sustain food webs throughout the winter. Thus, our results indicate that changes in winter temperatures will have important implications for river ecosystems during winter and the rest of the year. Key Points: Winter phytoplankton biomass was low across the river basin but comparable to summer in some areas and years Long‐term trends were not synchronous among river reaches and habitats Phytoplankton biomass increased during warmer winters across the basin