Phosphorus Dynamics and Availability in the Nearshore of Eastern Lake Erie: Insights From Oxygen Isotope Ratios of Phosphate

David C Depew,Geoffrey Koehler,Veronique Hiriart-Baer

doi:10.3389/fmars.2018.00215

David C Depew, Geoffrey Koehler + Show 1 more

Open Access

https://doi.org/10.3389/fmars.2018.00215

Copy DOI

Abstract

Blooms of filamentous benthic algae that plagued Lake Erie in the 1950s through 1970s were largely reduced through reductions of phosphorus (P) loading from point sources. Since the mid-1990s, these blooms have returned despite a period of relatively stable external P inputs. While increased loadings of dissolved P have been causally linked to cyanobacterial blooms in some parts of the lake, the impacts of ecosystem changes such as the effect of invasive species on nutrient cycling and availability have not been fully elucidated, leading to uncertainty as to the effectiveness of additional non-point P management actions. Here we use the oxygen isotope ratios (18OP) of phosphate in concert with measures of water quality along the northern shore of the east basin of Lake Erie to identify sources and pathways of P cycling and infer potential importance in relation to annual blooms of Cladophora that foul the shorelines of eastern Lake Erie. 18OP data indicate that potential external source signatures are rapidly overprinted by biological cycling of P by the plankton community and that much of the available phosphate in the nearshore waters is derived from hydrolysis of dissolved organic P compounds. Near the dreissenid-colonized lake bed, 18OP was persistently and significantly enriched in 18O relative to 18OP measured in surface waters and was similar to 18OP of phosphate excreted by dreissenid mussels in incubations. These results implicate dreissenid mussels as key agents in nearshore P cycling and highlight the importance of considering ecosystem changes in the development of nutrient management strategies designed to ameliorate symptoms of eutrophication.

Highlights

In the North American Laurentian Great Lakes, blooms of filamentous green algae were one of the most visible signs of cultural eutrophication in the early 1950s through to the late 1970s, in Lake Erie (Shear and Konasewich, 1975; Higgins et al, 2008)
No strong spatial patterns in dreissenid abundance or biomass were observed, and Cladophora biomass was high at the 1 m sites close to the Grand River, we do not have data for similar depths at greater distances to the west side of the river, which obscures the lack of spatial patterns observed in other years (D.Depew, unpubl.data)
Based on the nature and magnitude of deviations from δ18OP_EQ, δ18OP, we infer that internal recycling of dissolved organic P (DOP) by the plankton community is a dominant pathway determining the overall δ18OP of the available Pi pool in eastern Lake Erie

Summary

Introduction

In the North American Laurentian Great Lakes, blooms of filamentous green algae (mostly Cladophora sp.) were one of the most visible signs of cultural eutrophication in the early 1950s through to the late 1970s, in Lake Erie (Shear and Konasewich, 1975; Higgins et al, 2008). While there is general agreement reductions in P availability are the most practical means with which to reduce the extent and severity of Cladophora blooms (Auer et al, 2010), uncertainty remains as to the relative importance of direct P inputs, P circulating within the lake, and P potentially recycled in the benthic environment by dreissenid mussels (Bootsma et al, 2015). These uncertainties have confounded efforts to develop revised P loading targets for the east basin of Lake Erie to address Cladophora blooms (GLWQA, 2015)

Methods

Results

Discussion

Conclusion