Establishing Spatial and Temporal Patterns in Microcystis Sediment Seed Stock Viability and Their Relationship to Subsequent Bloom Development in Western Lake Erie

Abstract

This study assesses variation in the abundance and viability of sediment Microcystis vegetative seed stocks in Western Lake Erie across both seasons and years. Previous research suggests that lake sediment seed stocks can serve as inocula for reoccurring harmful algal blooms (HABs). However, there are few studies aimed at understanding the distribution, abundance, and viability of sediment seed stocks in Western Lake Erie and specifically how these variables are potentially related to past and subsequent bloom formation. We conducted a two-year study of vegetative seed stocks in the Western Lake Erie basin, the region where annual algal blooms generally develop. Sediment was collected from 16 sites within Western Lake Erie covering an area of 375 km2 with water column depths ranging from 3-9 meters. Sample collection occurred in November 2014, April 2015, November 2015, and April 2016. The total and potentially-toxic portions of Microcystis were determined using quantitative polymerase chain reaction. A series of laboratory experiments using lake sediment samples were conducted to assess the viability of Microcystis vegetative seed stocks. The abundance and viability of Microcystis vegetative seed stocks varied both spatially and temporally. Across all sampling periods, the abundance of total Microcystis in the sediment ranged from 6.6 x 104 to 1.7 x 109 cell equivalents g-1, and potentially-toxic Microcystis ranged from 1.4 x 103 to 4.7 x 106 cell equivalents g-1. The abundance of total Microcystis diminished significantly across winter with densities in the spring nearly 10 times less than the previous fall. No correlation was found between abundance at specific sites and sediment composition, depth, or distance offshore. Further, total sediment Microcystis abundance was not relatively larger in November 2015, even though 2015 yielded one of the largest blooms on record. However, a higher percentage of the sediment population in November 2015 was potentially-toxic across all sites, which may have been the result of the large nutrient loads and higher than normal in-lake nutrient concentrations. Culture experiments using sediment inocula and WC-Si growth media were used to examine potential viability of the sedimented cells. The sites with the greatest abundance of total and potentially-toxic Microcystis cells did not necessarily yield the most recruitment and growth over time, suggesting that abundance alone does not explain potential viability of sediment seed stocks. However, on average, the total abundance of cells in the grow-out flasks was more than twice the estimated amount present in the inocula, indicating that substantial growth occurred following recruitment from the sediment into the overlying water. Additional research will be needed to understand what specific factors influences the total contribution of Microcystis sediment seed stocks to recurrent annual blooms. However, numerical analysis suggests that sediment recruitment may have a significant impact on subsequent blooms, particularly when recruitment is paired with subsequent continual growth.