NUTRIENT RECYCLING

Sediment nutrient recycling is the process by which the accumulation of organic nutrient-rich sediment drives eutrophication.

This sediment creates a nutrient stockpile that is endlessly recycled, sustaining successive cycles of phytoplankton and invasive weed growth and die-off.

Sediment accumulates when algae and weed biomass dies off and decomposes, depleting oxygen from the water at the bottom and creating anaerobic conditions. In these anaerobic conditions, nutrient recycling rates and concentrations are elevated, completing a vicious circle of feedback that fuels more algae and weed growth.

Sediment nutrient recycling serves as the driving force behind declining water quality by shifting a water body’s core behavior from nutrient clearance and maintenance of water quality to nutrient recycling, eutrophication, and degradation of water quality.

Understanding and addressing sediment nutrient recycling is crucial for effectively managing and reversing eutrophication in aquatic ecosystems.

There are two main approaches to assessing the extent that sediment nutrient recycling is contributing to the eutrophic status of a water body:

1. Measure all Possible Chemistry Parameters.  This approach involves measuring a diverse set of water chemistry parameters, such as Total Phosphorus, Soluble Reactive Phosphorus (orthophosphate), Total Kjeldahl Nitrogen, Nitrate, Nitrite, Ammonia, and Biological Oxygen Demand (BOD). These parameters are second-order symptoms of nutrient recycling and indicate nutrient levels in the water.

2. Simple Logical Scientific Method.  However, because nutrient solubility, availability, and chemical form are highly correlated with oxygenation, measuring oxygenation and applying Sherlock Holmes-style inductive reasoning can provide a good understanding of the problem’s root cause.

3. Phycological Data Analysis: Analyzing phycological data offers insight into the degree of sediment nutrient recycling.

High phytoplankton biovolume and a greater predominance of cyanobacteria among the phytoplankton clearly indicate that sediment nutrient recycling and an abundance of recycled benthic nutrients are at play.

Increased algae levels, in general, show that nutrients are abundant.

An increase in cyanobacteria levels indicates that anoxic benthic conditions are ensuring nutrients are more available at the bottom, where cyanobacteria can control their buoyancy, dive down to preferentially access these nutrients, and outcompete floating algae.

Lastly, using bathymetry to analyze the surface area of sediment covered by hypoxic water quantifies the proportion of the lake bottom that is recycling sediment nutrients due to hypoxic conditions.

In summary, sediment nutrient recycling is a critical factor in the development and persistence of eutrophication in aquatic ecosystems.

By understanding the importance of this process and employing effective measurement techniques, such as oxygenation monitoring, phycological data analysis, and bathymetric assessment, lake managers can better target the root causes of eutrophication and develop more effective strategies for restoring and maintaining the health of these vital water resources.