Evaluating Phosphorus Limitation from the Maumee and Sandusky Rivers into Lake Erie Heather R. Kirkpatrick, Curtis C. Clevinger, Moumita M. Moitra, Darren L. Bade, Joseph D. Conroy, David A. Culver, William J. Edwards, Douglas D. Kane

Lake Erie & Eutrophication 1960’s: Harmful Algal Blooms (HABs); extreme eutrophication; hypolimnetic hypoxia 1972: Great Lakes Water Quality Agreement (GLWQA) signed; reduced phosphorus (P) entering Lake Erie; P detergents banned Late 1970’s-1980’s: Reductions in P recorded; decreased HABs and hypoxia Mid-1990’s: re-eutrophication of Lake Erie; increases in dissolved P; return of HABs and hypoxia Sandusky Bay in April (OhioLink Landsat 7)

P-limitation Paradigm P is considered most often limiting in freshwater Nitrogen fixation should reduce the occurrence of N limitation Managing for N may favor N limitation and N fixing cyanobacteria

P-limitation Demand of P by algae exceeds supply  Other factors are not limiting When P supply > demand  Another factor is limiting

Overall Project Goals To directly assess the role of P in facilitating HABs by determining: 1. How Microcystis uses the bioavailable portion of the total P? 2. Where in these systems blooms are triggered (upstream, nearshore or offshore)?

Specific Project Goals To assess conditions of P-limitation by determining: 1. Where and when does P-limitation occur? 2. What are the causes and consequences of P-limitation (or lack thereof)?

Methods Algal Phosphorus Debt (P-debt): - Amount of phosphate incorporated in 24 hours in dark - The more “phosphate starved” algae are the more phosphate is taken up - Scaled to algal biomass - Values above µmol P/ µg chl are considered P limited Phosphate Turnover Time (P-turnover): - Fast turnover times (less than 60 minutes) indicate P- limitation - Fast turnover times could be caused by a small phosphate pool or high demand by algae Healey & Hendzel 1979 Heath, R.T. 1986

Methods Chlorophyll a Secchi depth Total phosphorus Nitrate Ammonia

Sandusky System Results

Indicates agreement with P-turnover data LIMITED NOT LIMITED

Maumee System Results

Indicates agreement with P-turnover data LIMITED NOT LIMITED

Summary Sandusky system limited in 40% of the samples Maumee system limited in 70% of the samples Lake and tributary sites variable Sandusky Bay generally not P-limited Maumee River generally not P-limited

Summary Periods of high chlorophyll and low nitrate concentrations at Sandusky Bay and Maumee River sites Since P is generally not limiting at these sites, other factors (e.g. nitrogen) may be limiting

Conclusions P-limitation not ubiquitous Nitrogen may also be important in limiting algal growth Although we have potential nitrogen limitation, most cyanobacteria were not N fixing Management considering multiple factors may be more successful

Acknowledgements Funding: Lake Erie Protection Fund The Ohio Academy of Science Field collection crew Joshua Smith Dominique Guyton

LIMITED NOT LIMITED

LIMITED