In this article, principles of phosphorus retention and phosphorus release at the sediment-water interface in lakes are reviewed. New results and hypotheses are discussed in relation to older models of phosphorus exchange between sediments and water. The fractional composition of sedimentary phosphorus is discussed as a tool for interpretation of different retention mechanisms. Special emphasis is given to the impact ofbiological, particularly microbial, processes on phosphorus exchange across the sediment-water interface and to the significance of biologically induced CaCO, precipitation to phosphorus retention in calcareous lakes,

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The recent literature on the bioavailability of different forms of P in freshwater systems is reviewed. Bioavailable P is defined as the sum of immediately available P and the P that can be transformed into an available form by naturally occurring processes. Methods used to estimate the bioavailable P pool, which vary between studies largely depending on the time perspective applied, are critically evaluated. Most studies on particulate P aim to determine the potentially available P pool. Potential bioavailability of particulate P is normally analysed in bioassays with algal yield determinations and the available P fraction is characterized from interpretations of results of sequential chemical extractions. NaOHextractable P is in most studies the most algal-available P fraction. For soil samples and 'tributary water particulate matter, NaOH-P has often been found to be equal to algal extractable P. In other studies depletions of NaOH-P have accounted for the algal P uptake, but only a minor proportion of the fraction has been utilized. Organic P in lake water particulate matter and bed sediments of eutrophic lakes can also be algal-available to a significant extent.Studies on the bioavailability of dissolved P have often been concerned with immediate availability, or the minimum amount of available P. Such studies need other types of experimental design and normally assays with radiotracers are used. Immediately available P is frequently found to be less than P chemically assessed as dissolved reactive P (DRP) at low (< 10 pg DRP * l-') concentrations. However, immediate availability may also approach or exceed DRP concentrations, especially at higher concentrations. Potential bioavailability, assayed as for particulate P, may generally render higher bioavailability than P assayed as immediately available. Large fractions of dissolved P remain unutilized and are primarily found in the high molecular weight fraction of dissolved P. Introductionthe total P load on lakes will be available to the lake biota for production of organic matter. In Empirical relations between phosphorus loading, recent years increased attention has therefore phosphorus concentrations and algal biomasses been paid to determinations of the bioavailable or in lakes are normally based on total phosphorus, algal-available proportion of P from different cf. the OECD report on eutrophication of waters sources, with the aim of improving understanding (OECD, 1982). However, it is well known, and of P cycling in lakes, refining empirical models or also stated in the OCED report, than only part of improving lake management strategies.

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Feeding by Chironomusplumosus (L.) larvae on sedimentary microbiota was studied in eutrophic Vallentunasjiin, where microbial biomass constitutes a large portion of sedimentary organic matter. The life history of C. plumosus consisted of two univoltine cohorts with an overall production of 1.4 g ash-free dry wt m-2. Gut concentrations of bacteria, Microcystis, and Melosira were several times higher than sedimentary concentrations, indicating that the larvae are foraging selectively. Assimilation of bacteria and newly scdimentcd Melosira cells on average accounted for 1 I and 49% of the estimated carbon requirement (production + respiration) of the larvae. Digestive efficiencies for bacteria and Melosira seasonally averaged 43 and 34%. "Over-wintering" Microcystis colonies, which constitute the dominant proportion of microbial biomass in the sediments, were used to a small extent if at all as a carbon source by the chironomids. Ifa decrease in autofluorescence intensity of cells during gut passage is indicative of partial digestion of the cells, Microcystis was used with a digestive efficiency of 9% and accounted for -10% of the carbon requirements of the larvae. On the basis of ambient Chironomus densities, gut passage time, enrichment factor (gut vs. sedimentary density of bacteria), and digestive efficiency, chironomid grazing is not a dominant sink for sedimentary bacteria in Vallentunasjon. The feeding activity of Chironomus larvae decreased bacterial densities but increased the cell-specific production in laboratory experiments. The decrease was not proportional to grazer density, indicating that indirect effects of the chironomids (on sedimentary structure and chemistry) also affect the abundance of bacteria.