Flux of Matter

Nutrient Cycles and Spiraling

The limnological study of standing waters has always been dominated by a conceptual model of closed ecosystems, in

Coarse particulate organic matter Dissolved organic matter

Light r

Microbes

Microbes

Dissolved organic matter

Flocculation

Macroproducers

Dissolved organic matter

Microproducers

Flocculation

Microbes

Shredders

Fine particulate organic matter

Fine particulate organic matter

Shredders

Figure 3 Conceptual model of invertebrate functional feeding groups and their food resources in a small, forested stream ecosystem. Modified from Cummins KW (1974) Structure and function of stream ecosystems. Bioscience 24: 631-641.

Table 1 Numbers of families and functional group assignment of some representative orders of benthic macroinvertebrates in running-water ecosystems

Number of families by dominant functional feeding groups

Total number of Filtering Gathering Filamentous algal

Order families Shredders Scrapers collectors collectors Predators piercers

Ephemeroptera 21 2 5 10 4

Plecoptera 9 6 3

Trichoptera 26 5 8 6 4 2 1

which nutrients recycle seasonally, totally within the system. The unidirectional flow of running waters necessitated modifying this view of closed cycles in lakes to an open-cycle model; that is, the open nutrient cycles in streams and rivers follow a spiraling pattern in which nutrients generated (or delivered) at one point along a stream or river complete the recycling to their initial state at a displaced location downstream (Figure 4). Total spiral length represents the sum of the distance traveled by an element as an inorganic solute until its uptake by the biota, plus the distance traveled within the biota until its release back into the water column. If nutrients such as nitrogen or phosphorous are cycled rapidly, the spirals are 'tight', that is, the downstream completion of the cycle is short. If cycling is slow, the closing of the loop is displaced a longer distance downstream and the spirals are more open. The tighter the spiraling cycling loops, the more retentive (conservative) is the stream or river reach.

Transport and Storage of OM

The transport and storage of OM in running-water ecosystems involves complex interactions between (1) the state of the OM, (2) the source of the OM, and (3) the physical, chemical, and biological retention potential for any given reach of stream or river.

State of the OM

Three broad categories of OM are dissolved (DOM, size range <0.45 mm), fine particles (FPOM, size range >0.45 mm to 1 mm), and coarse particles (CPOM, size range >1 mm). While FPOM particles are colonized

Mechanism

Effect on nutrient cycling

Retention

Biological activity

Rate of recycling

Distance between spiral loops

Ecosystem response to nutrient addition

Ecosystem stability

High

73 iS

Fast

Short

Short

73 iS

Stream flow

Distance between loops

Stream flow

Conservative

Distance between loops

High

Slow

Short

Slow

Short

Storing

High

Fast

High

Long

Fast

Long

Intermediately conservative < A but > D

Slow

Long

Slow

Long

Exporting

Figure 4 Nutrient spiraling depicted as the effects of different interactions between the distance of downstream movement (velocity x time) and measures of biological activity such as metabolism by benthic microbes. Modified from Minshall GW, Petersen RC, Cummins KW, etal. (1983) Interbiome comparison of stream ecosystem dynamics. Ecological Monographs 53: 1-25.

primarily on the surface by bacteria, CPOM is colonized by fungus, bacteria, and microzoans that penetrate the matrix of the material. Aquatic hyphomycete fungi usually penetrate the CPOM leaf and needle litter first. Bacteria and microzoans follow the fungal hyphal tracks into the matrix of the CPOM. The OM in solution (DOM) includes a full range of molecules from simple very labile ones such as sugars and amino acids to complex recalcitrant compounds such as phenolic compounds.

Sources of the OM

A major source of OM in streams (orders 0-5) is the riparian zone. This border of stream-side vegetation produces litter (e.g., leaves, needles, bud and flower scales, seeds and fruits, small wood and bark) that enters on a seasonal schedule depending upon the relative proportions of deciduous and evergreen species. Other sources of OM are solutions and particles from bank erosion, DOM leachates from litter, exudates, and leachates from periphytic algae and vascular aquatic plants together with their physical fragmentation and mortality.

Physical, chemical, and biological retention potential

The retention of DOM involves physical flocculation of the OM in solution with divalent cations, such at Ca++, and biological uptake by resident bacteria and fungi. Chemical reactions between the smaller molecular weight organic compounds may precede the physical complexing with cations. The rate and extent of biological uptake of DOM depends upon factors such as the lability or recalcitrance of the compounds, density and composition of the microbial flora, and water temperature. These mechanisms that convert DOM to FPOM, flocculation and microbial uptake, are quite important ecosystem processes. The conversion of DOM in solution to particles significantly increases the retention of the OM. The difference in the efficiency of retention of OM between soft, stained water streams and hard, clear water streams accounts in part for the greater productivity of the latter. The POM that results from the conversion of DOM is more likely to remain in a given reach of stream or river and enter into trophic pathways.

Retention of POM depends upon channel geomor-phology. Large wood debris (LWD), branches and exposed bank roots, coarse sediments, backwaters, side channels, and settling pools are all important retention features. For any given reach of stream or river, a major source of OM is transport from upstream. In addition, OM is retained when bankfull-flow is exceeded and material is deposited on the upper banks or on the floodplain. OM is returned to the channel when water levels recede. Whether these off-channel areas serve as sources or sinks for OM over an annual cycle depends upon the configuration of the upper banks and flood-plains and the patterns of the flood flows. The general fertility of floodplains suggests that they are largely sinks.

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