A regime shift from macrophyte to phytoplankton dominance enhances carbon burial in a shallow, eutrophic lake

Abstract

Abstract. Ecological regime shifts and carbon cycling in aquatic systems have both been subject to increasing attention in recent years, yet the direct connection between these topics has remained poorly understood. A four-fold increase in sedimentation rates was observed within the past 50 years in a shallow eutrophic lake with no surface in-or outflows. This change coincided with an ecological regime shift involving the complete loss of submerged macrophytes, leading to a more turbid, phytoplanktondominated state. To determine whether the increase in carbon (C) burial resulted from a comprehensive transformation of C cycling pathways in parallel to this regime shift, we compared the annual C balances (mass balance and ecosystem budget) of this turbid lake to a similar nearby lake with submerged macrophytes, a higher transparency, and similar nutrient concentrations. C balances indicated that roughly 80% of the C input was permanently buried in the turbid lake sediments, compared to 40% in the clearer macrophyte-dominated lake. This was due to a higher measured C burial efficiency in the turbid lake, which could be explained by lower benthic C mineralization rates. These lower mineralization rates were associated with a decrease in benthic oxygen availability coinciding with the loss of submerged macrophytes. In contrast to previous assumptions that a regime shift to phytoplankton dominance decreases lake heterotrophy by boosting whole-lake primary production, our results suggest that an equivalent net metabolic shift may also result from lower C mineralization rates in a shallow, turbid lake. The widespread occurrence of such shifts may thus fundamentally alter the role of shallow lakes in the global C cycle, away from channeling terrestrial C to the atmosphere and towards burying an increasing amount of C.

Multi-Elements in Waters and Sediments of Shallow Lakes: Relationships with Water, Sediment, and Watershed Characteristics

“…Some research has explored how nutrients in lakes relate to nearby land use (Fraterrigo and Downing 2008; Wezel et al 2013) and phytoplankton biomass (Grace et al 2010; Borics et al 2013). Related studies of shallow lakes have focused on identifying factors responsible for water-clarity regime shifts, variations in phytoplankton biomass, and macrophyte abundance (Schippers et al 2006; Zimmer et al 2009; Bayley et al 2013; Brothers et al 2013; Llames et al 2013; Zhang et al 2013; Robin et al 2014). As mentioned previously, our study adds to these earlier works by examining concentrations of multiple elements, including the rare earth elements in the waters and sediments of shallow lakes.…”

“…We hypothesized that land use within watersheds, along with water and sediment characteristics, influenced element concentrations in waters and sediments of shallow lakes. Earlier shallow lake studies focused on regime shifts, carbon, phosphorus and nitrogen cycling, phytoplankton biomass, and macrophyte abundance (Scheffer and Jeppesen 1998; Hansel-Welch et al 2003; Schippers et al 2006; Bayley et al 2007, 2013; Zimmer et al 2009; Brothers et al 2013; Llames et al 2013; Zhang et al 2013; Robin et al 2014). Most of these earlier works focused on influences of various environmental variables on water quality and chemistry, but few have considered physical or chemical aspects of shallow lake sediments (Barko and James 1998; Sahuquillo et al 2012; Kissoon et al 2013; Wu et al 2014).…”

Benthic carbon is inefficiently transferred in the food webs of two eutrophic shallow lakes

“…To estimate the biomass and production of fish, abundance data were derived from a 5-day mark-recapture approach conducted in October 2011, as described in Brothers, Hilt, Attermeyer, et al (2013) and Mehner et al (2016). Briefly, we caught fish with an electrofishing device (Bretschneider Spezialelektronik, Breitenbrunn, Germany) and tagged them using coded wire tags (Northwest Marine Technology, Inc., U.S.A.) that were inserted into the snout region.…”

“…These were estimated from a standardised fishing campaign using four Nordic multimesh gillnets which were installed perpendicular to the shoreline from dusk until dawn, and additional standardised electrofishing (applying 15 dips for 15 s at randomly chosen locations) (see Brothers, Hilt, Attermeyer, et al, 2013;Mehner et al, 2016 for details). These were estimated from a standardised fishing campaign using four Nordic multimesh gillnets which were installed perpendicular to the shoreline from dusk until dawn, and additional standardised electrofishing (applying 15 dips for 15 s at randomly chosen locations) (see Brothers, Hilt, Attermeyer, et al, 2013;Mehner et al, 2016 for details).…”

Whole‐lake experiments reveal the fate of terrestrial particulate organic carbon in benthic food webs of shallow lakes

“…Furthermore, we calculated the input by tDOC (terrestrial dissolved organic carbon; see Brothers et al 2013a, b) and the tPOC influx from alder and maize (see Appendix A for details of methods) to document that the pulsed POC input from maize was relatively minor in comparison with the autochthonous carbon fixation in the lakes.…”