Fixing Sediment Phosphorus

Internal phosphorus (P) loading from lake sediments is a critical issue, serving as a significant driver behind harmful algal blooms (HABs) within many lake systems. Of the 100+ most problematic mat-forming filamentous and planktonic cyanobacteria in lakes, all are directly linked to lake sediments. Their cell resting stages (akinetes) germinate and grow into full colonies in the sediments when light, temperature, and nutrients become ideal. The overall growth or biomass observed once they “pop up” in the surface waters can be directly linked to the amount of available sediment P in many lakes and ponds. Once internal P loading exceeds 25% of the total P load to a lake, it is the primary source governing surface algal blooms.

Addressing P within the sediments starts with knowing how much sediment P needs to be addressed. This can be achieved by collecting surficial sediments from the “active” P layer of sediment within a lake and then performing a laboratory P fractionation to determine the amount of reactive P (labile and reductant P) in collected sediments. For most lakes, the goal is to reduce sediment mobile P below 50 mg/kg. However, this requires accurately calculating available P and properly selecting and dosing P removal products. Three critical factors that are often overlooked in this process include:

1. “Active” Sediment Depth – estimating the mass of P that needs to be treated is done by multiplying the amount of mobile sediment P by the target depth of sediment to be treated, often referred to as the active sediment layer (e.g., 0-10cm would be a 10 cm slice) to yield g/m2 of P. The general rule of thumb is to assume at least 10 cm of active sediment (Cooke et al. 2016; Wagner 2017). Increasing this should be considered in shallow lakes subject to motorized boating or lakes with confirmed rough fish like common carp. For example, common carp will mix sediments on average 13 cm down with a max of 28 cm (Huser et al. 2016)

2. Sediment-Specific Gravity and Percent Solids – Multiplying the percent solids by the lake’s sediment-specific gravity is needed to calculate the mass of available P. In general, the denser and more compact lake sediments are, the more mass of P that is available that needs to be treated.  Specific gravity, by definition, is the ratio of a substance’s density to the density of water. It’s a dimensionless number used to compare the relative density of a substance to water and has no units.

3. The Specific Gravity of Products – Products applied to lake sediments that aim to address sediment P must stay in the intended active sediment zone (~10 cm) to work effectively and have long-lasting results. This means products with higher specific gravities will be buried faster and not last as long as products with lesser specific gravity. Products using clay (e.g., bentonite) often have higher specific gravities meaning they will be buried faster via movement towards deeper, denser sediment. Liquid products like MetaFloc or tablet products like Muckbiotics have specific gravities similar to most lake sediments which keeps them from being buried quickly and extends their duration of benefits.

The Natural Lake Biosciences Aquatic Research Laboratory at the University of Wisconsin Research Park provides various services, including water quality, algae, and sediment analysis. Stay up to date with the newest laboratory services coming in 2025!

Figure 4. Picture of Patrick with Sediment Core