Macrophytes enhance reach-scale metabolism on a daily, seasonal and annual basis in agricultural lowland streams

Abstract

In many lowland streams, macrophytes are highly abundant and play a key role in ecosystem structure and function. However, no studies on annual stream metabolism have been conducted in streams with significant macrophyte abundance, despite the well-known effect on both gross primary production (GPP) and ecosystem respiration (ER). Macrophyte abundance in temperate streams is strongly seasonal, with highest biomass during summer and lowest during winter. We expected that this phenological pattern would drive annual fluctuations in GPP and ER. We measured daily metabolism for one year in two stream reaches, one with and one without macrophytes. Our results demonstrated that annual, aggregated GPP and ER were 2.2 and 1.3 times higher in the macrophyte reach. Furthermore, while daily GPP was the same between the two reaches during winter where biomass was negligible, GPP was higher during spring, summer and fall for the macrophyte reach. The range in daily ER was more constrained during summer, but more variable during fall and winter in the macrophyte reach relative to the non-macrophyte reach. Macrophyte abundance and chlorophyll-a controlled 80% of the variation in annual GPP for the macrophyte reach. Similarly, 63% of the variation in annual ER was controlled by macrophyte abundance together with discharge in the macrophyte reach. Although macrophytes enhanced GPP on an annual and seasonal time scale in agricultural lowland streams, both reaches were heterotrophic (i.e., GPP < ER) reflecting high organic matter supply from the landscape and in-stream retention and decomposition of organic matter within the macrophyte beds.