Phosphorus Fractionation Sediment Test

Overview

This is a sequential extraction that separates sediment phosphorus by mobility instead of reporting it as a single number. Ten parameters come back: total phosphorus, plus the loosely bound, iron-bound, aluminum-bound, calcium-bound, biogenic, and organic fractions, plus total organic carbon, total solids, and percent water. It runs the Rydin and Welch scheme, with a reporting limit of 2 mg/kg on the phosphorus fractions.

The distinction it draws is the one that decides whether an in-lake treatment is worth doing. Total sediment phosphorus tells you the size of the inventory. Fractionation tells you how much of that inventory can actually return to the water column, which is a different and usually much smaller number.

Note on methodology: Rydin and Welch is a peer-reviewed limnological extraction scheme, not an EPA-approved method. It is the standard approach for this question in lake management practice, and results are intended for management planning rather than regulatory determination.

What problem does this solve for our lake?

Internal loading is the reason lakes fail to improve after the watershed has been cleaned up. Phosphorus deposited over decades sits in the sediment, and when the water above it runs out of oxygen in late summer, some of it comes back. In many older lakes internal loading is the largest single phosphorus source, and it is invisible from the surface. What each fraction means: • Loosely bound — pore water and exchangeable phosphorus. Immediately mobile, released by mixing, resuspension, or boat wakes in shallow water. Small in mass, fast in effect. • Iron-bound — the redox-sensitive fraction, and the one that matters most. Phosphorus held on iron oxides stays put while oxygen is present and is released when the hypolimnion goes anoxic. This is the engine of classic internal loading, and the fraction an alum treatment is designed to intercept. • Aluminum-bound — effectively permanent burial. Not released regardless of oxygen conditions. This fraction is also the record of any previous alum treatment, so it tells you whether the last one is holding. • Calcium-bound — apatite phosphorus, locked to calcium carbonate and stable on any management timescale. This fraction routinely dominates in hard-water marl lakes, which describes a great many lakes across the upper Midwest. A lake with a large total phosphorus inventory that is mostly calcium-bound is a poor candidate for chemical treatment no matter how alarming the total looks. • Biogenic and organic — phosphorus held in organic matter. The biogenic pool is released slowly as that material mineralizes; the residual organic pool is largely refractory. A long-term supply rather than a seasonal one. • Total phosphorus — the inventory, and the least useful number on its own. • Total organic carbon, total solids, and percent water — the supporting measurements. Total organic carbon matters more than it looks: organic matter is what consumes oxygen at the sediment surface, and that oxygen loss is precisely what releases the iron-bound fraction. Solids and percent water put every result on a comparable dry-weight basis. The decision this drives is concrete. If most of the phosphorus is calcium-bound, aluminum-bound, or refractory organic, an alum treatment buys very little and the money is better spent in the watershed. If a large mobile and iron-bound fraction sits under a lake that stratifies and goes anoxic, an alum or aeration project is targeting something real. Against a treatment that commonly runs into six figures, a $434 test that tells you whether to proceed at all is the cheapest line in the project budget.

What's included?

• A complete Phosphorus Fractionation Sediment test kit shipped directly to you
• All sample containers and materials required for clean, contamination-free handling
• Step-by-step collection and preservation instructions
• Prepaid 2-day return shipping to our partner laboratory
• Full chain of custody and professional lab handling
• Results loaded automatically into your lake's account
• Note on methodology: this test runs the Rydin and Welch sequential extraction, a peer-reviewed limnological method rather than an EPA-approved one. It is the standard approach for assessing internal loading potential, and results are intended for management planning, not regulatory determination.

What's not included

• A sediment corer, Ekman dredge, or Ponar dredge. The collection device is NOT included and this test cannot be done with a grab bottle.
• Sample collection. You collect the sample; we supply the containers and instructions.
• Boat access over the deep basin, which is where the sample needs to come from
• Water column phosphorus. This is sediment only — order the Total Phosphorus Test or the Nutrient Profile Surface Water Test for the water.
• A measured release rate. This test characterizes release potential; it does not measure an actual phosphorus flux, which requires core incubation.
• Sediment metals or organic contaminants
• An alum dose calculation or a dredging volume estimate. Those are engineering deliverables — the Boathouse can help you scope them.

Requirements

• A SEDIMENT sample, not a water sample. This test requires a core or a dredge grab from the lakebed. A grab bottle of water will not produce a usable sample and the kit does not include a collection device.
• A sediment corer or an Ekman or Ponar dredge, which you supply or borrow. A corer preserves the vertical layering; a dredge does not. Record which you used — it changes how the result should be read.
• A sample from the depositional zone, meaning the deepest basin where fine material settles, not the shoreline and not sand or gravel
• The top 10 cm of sediment, which is the biologically and chemically active layer
• The jar filled with minimal headspace and minimal air exposure
• Kept cold and dark, shipped the same day
• Depth and coordinates recorded, so the site can be reoccupied in future years
• A Plus, Lake Admin, or PRO subscription to have results loaded into your Lake Pulse account
• Questions about site selection, timing, or which test fits your lake? Contact the Lake Pulse Boathouse before you order.