Primary production modeling identifies restoration targets for shifting shallow, eutrophic lakes to clear-water regimes

Abstract

Benthic primary production (BPP) plays an important functional role in lakes, improving water quality by stabilizing clear-water regimes. Shallow, eutrophic lakes lacking BPP communities can be difficult to restore because self-stabilizing feedbacks of phytoplankton dominance can impede the establishment of BPP. BPP in lakes is light limited, and ecosystem models can provide guidance for determining the water clarity necessary to reestablish BPP dominance. We developed a multi-tiered framework for the restoration of shallow, eutrophic lakes that incorporates multiple turbidity sources, lake morphometry, and water-level fluctuations to determine waterclarity thresholds above which BPP dominance may be established. We present a case study applying this restoration target framework to a large, shallow lake (Utah Lake, Utah, USA), where water clarity was greatly impeded by sediment resuspension and high algal biomass. Our analysis, which used commonly available lake-monitoring data, indicated that a return to BPP dominance is possible in Utah Lake, particularly if external nutrient loading to its shallow, wind-protected bays is reduced. Our novel framework incorporates regime shift theory to improve shallow, eutrophic lake restoration efforts. By incorporating ecological feedbacks when identifying the restoration targets necessary for returning a lake to a self-stabilizing, clear-water regime, this restoration target framework offers economical and logistical advantages over strategies that focus solely on phytoplankton management or fish biomanipulation.

Anthropogenic forcing leads to an abrupt shift to phytoplankton dominance in a shallow eutrophic lake

“…An interpretation of abundant macrophytes within Goshen Bay in the past is further supported by the detection of sed DNA markers for emergent hardstem bulrush, a taxon indicative of good water quality conditions (Croft & Chow‐Fraser, 2007) and beneficial to littoral fish communities (Reed & Pereira, 2009). The presence of emergent macrophytes would have reduced sediment resuspension by stabilising sediments, which in turn may promote the light conditions necessary for submerged macrophyte and periphyton growth (King et al., 2023). Further, the presence of molecular markers of hardstem bulrush in older samples may be indicative of lower lake‐levels during that interval, as climate‐driven water‐level fluctuations and abrupt shifts in shallow lake structure are often closely intertwined (Kong et al., 2017; Van Geest et al., 2007).…”

“…Differences in the patterns of Chl a : Pheo a among coring sites may reflect local variation in water‐column depth relative to that of light penetration, such that routinely low pigment ratios at Bird Island are consistent with the greater depth, and presumptive lack of macrophytes suppressing sediment turbidity at this locale. The establishment of benthic primary producers at the Bird Island coring site would require enhanced water clarity compared to levels observed today (King et al., 2023).…”

“…Efforts have been underway since 2010 to reduce the abundance of carp, re-establish submerged macrophytes, and re-introduce native fish species (Walsworth et al, 2020). Today, the lake's water clarity is impeded by high sediment resuspension and phytoplankton production, which together control lake productivity by limiting benthic primary production (King et al, 2023). Additionally, the region has experienced near-exponential population growth over the past 100 years, including c. 30% population growth since 2010 (2020 population = 653,600;World Population Review, 2022).…”

Cyanobacteria in the Anthropocene: Synanthropism forged in an era of global change

“…Instability is a key hallmark of the Anthropocene era. Cyanobacteria appear to have reasserted their dominance, with evidence from sediment records indicating a shift from eukaryotic predominance (e.g., diatoms), with cyanobacteria abundance outpacing diatoms in contemporary times (from around 1945 onwards) (Engstrom et al, 1985; King et al, 2024; Taranu et al, 2015). Predictors of global change point to a “blue‐green” future, which aligns with current observations of cyanobacteria expanding geographically and blooms of greater magnitude and duration in response to anthropogenic ecological change (Erratt, Creed, Lobb, et al, 2023; Huisman et al, 2018).…”