Seasonal Influence of Light Availability on Metabolism in an Urban, Spring-fed River

Stehle, Matthew
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In aquatic ecosystems, temperature and light are the primary drivers of ecosystem metabolism, which is comprised of gross primary production (GPP) and ecosystem respiration (ER). However, light availability and temperature are generally seasonally synchronous in aquatic systems outside of the tropics, thus it is difficult to assess the independent influence of each parameter on ecosystem metabolism. I examined the effects of seasonal light availability on both GPP and ER, macrophyte community dynamics, and autotrophic metabolism in the San Marcos River, a physicochemically stable spring-fed system with high perennial macrophyte biomass. I estimated GPP, ER, and net ecosystem production (NEP) for spring, summer, and winter seasons in the upper San Marcos River. Using high-resolution drone imagery and in-field measurements, I determined seasonal biovolume (m3) across four reaches of varying recreational intensity. Finally, I used metabolic chambers to estimate seasonal GPP, respiration (R), and net primary production (NPP) of three macrophyte species and plastic plant analogues of increasing morphological complexity to determine how structural complexity influences metabolism of macrophytes and their epiphyte communities. I found that both GPP and ER in the upper San Marcos River were strongly correlated with seasonal light availability, indicating autotrophic respiration as a substantial portion of ER. I found that reach-scale macrophyte coverage and biovolume are largely controlled by recreational activity and not seasonal light availability. Finally, macrophyte and epiphyte metabolism were different across seasons and morphotypes, with the most structurally complex macrophytes exhibiting the greatest epiphyte metabolism. Overall, this study indicates that ecosystem metabolism in thermally stable spring-fed rivers is largely driven by light limitation of autotrophic biomass and that human recreational intensity influences ecosystem functioning.
Ecosystem metabolism, Springs, Ecology
Stehle, M. (2022). <i>Seasonal influence of light availability on metabolism in an urban, spring-fed river</i> (Unpublished thesis). Texas State University, San Marcos, Texas.