Organic Matter

Abstract

Organic matter in aquatic ecosystems ranges from dissolved organic compounds to large aggregates of particulate organic matter and from living to dead material. Most of the organic matter, whether dissolved or particulate, is detritus (i.e., organic matter from dead organisms). Metabolism of the organic matter and interactions of this material chemically and biologically are, to a significant extent, governed by the size and chemical composition of the organic matter. Little, if any, dissolved and colloidal organic matter is utilized by aquatic animals directly, whereas particulate organic matter of a given size range may be a major food source. Decomposition of dissolved organic matter results in gaseous end products; particulate organic matter must be converted enzymatically to soluble organic compounds prior to biochemical degradation to gaseous products.

Nearly all of the organic carbon in natural waters is in the form of dissolved organic carbon (DOC), colloidal organic carbon (COC), and dead particulate organic carbon (POC). This quantity of detrital (nonliving) organic carbon greatly exceeds that amount of carbon in all of the living biota combined. A ratio of ca. 10:1 for DOC/COC to POC is almost universal in the open water of both lake and stream ecosystems (Wetzel, 1984, 1995).

DOC/COC may be separated from POC by sedimentation, centrifugation, and filtration. Historically, DOC has been defined rather arbitrarily in most studies by the practical limit of filtration of natural water—that amount of organic matter retained on a filter of 0.5-μm pore size is the POC; the organic carbon of the filtrate is the DOC/COC fraction (cf., Gustafsson and Gschwend, 1997). The filter-passing materials are a mixture of truly dissolved organic compounds and of macro-molecular colloids of a size of ca. 300–2000 Da to several thousand Kda (0.7–2 nm). Separation of dissolved/colloidal fractions from particulate components on the basis of size is not always consistent with thermodynamic chemical properties and reactivities. However, the physical separation described in this exercise, where the dissolved and colloidal components are not significantly affected by gravitational settling, is reasonably consistent with chemical delineations and is characteristic of transport in solution. Measured DOC concentrations, therefore, may contain a significant colloidal fraction in addition to truly dissolved organic carbon.

Particulate organic carbon of a size greater than 0.5 μm can originate from many sources, such as photosynthetic production of algae and aquatic plants, fragmentation of larger particles, animal maceration and egestion, microbial processes, flocculation of dissolved organic matter, and terrestrial inputs. Particulate organic matter (POM) is further differentiated on the basis of size into fine particulate organic matter (FPOM;>0.5μm to <1000μm or 1.0 mm) and coarse particulate organic matter (CPOM; any organic particle >1.0mm). Each of these categories is further differentiated into smaller categories (cf. Lamberti and Gregory, 1996; Wallace and Grubaugh, 1996). Determinations of the average organic carbon content of the POM or fractions of POM allow an estimate of the amount of organic carbon potentially available for metabolism by organisms. Quantification of the amount of organic carbon in POM or DOM does not mean that these organic compounds are available, however, for immediate hydrolysis and assimilation, because of the large variability in organic composition.

In this exercise, the amounts and distribution of DOC/COC and POC will be evaluated in the pelagic zone of a lake. Variations in a stream ecosystem are analyzed in a subsequent exercise (Exercise 24). Organic nitrogen and phosphorus compounds have been discussed earlier (Exercise 7).