Internal Loading of Nitrogen (N) and Phosphorus (P), Reduced N Forms, and Periodic Mixing Support Cyanobacterial Harmful Algal Blooms (HABs) in Shallow, Eutrophic Honeoye Lake (New York, USA)

Cyanobacteria are important primary producers, but large cyanobacterial harmful algal blooms (HABs) have many negative ecological and health impacts and are becoming increasingly common. Honeoye Lake (New York, USA) is a shallow, eutrophic lake characterized by increasingly frequent HABs. Nitrogen (N) and phosphorus (P) loads often drive HABs in lakes, and sediment processes can contribute to N removal (e.g., denitrification) or loading (e.g., N fixation, remineralization). Sediment cores and lake water were collected during May–October (2016–2018) at two sites and incubated with no amendments (controls) or 15N stable isotopes to measure sediment nutrient fluxes and N cycling dynamics in Honeoye Lake. Sediments were a strong source of ammonium (NH4+; 200 ± 56 µmol N m-2 hr-1) and soluble reactive P (SRP; 1.60 ± 0.67 µmol P m-2 hr-1). Internal loading of NH4+ was greater than previous estimates of external and internal TN loads. In situ denitrification (mean 17 ± 7 µmol N m-2 hr-1) was the main N removal pathway but was limited by NO3- availability and a lack of nitrification (mean 0.007 ± 0.002 µmol N L-1 hr-1). Potential dissimilatory nitrate reduction to ammonium rates (DNRA; 30 ± 11 µmol N m-2 hr-1) suggested sediments may play an important role in internal loading and recycling of N. Water column NH4+ uptake (mean 0.23 ± 0.02 µmol N L-1 hr-1) rates indicated high NH4+ demand, with only 50% of potential uptake being supplied by regeneration (mean 0.11 ± 0.01 µmol N L-1 hr-1) within the water column, while the other 50% can be accounted for from sediment NH4+ loading. Scaling these rates to the whole lake area suggests internal loads of bioavailable N and P are greater than external loads and promote primary productivity and HABs within Honeoye Lake. Shallow lake sediments can be a significant source of reduced N and SRP, which can be mixed periodically supporting HABs. N loads dominated by chemically reduced forms may limit denitrification and favor non-N-fixing cyanobacteria, as well as internal recycling pathways that retain N. An increased focus on reduction of N, along with P, is necessary to prevent increasing cyanobacterial HABs