Introduction

The length of food chains and complexity of food webs in aquatic systems - which includes fresh, marine, and brackish waters - are set by the productivity of the autotrophs (i.e., photosynthesizing organisms like phytoplankton, epiphytes, macroalgae, and water plants) and by the number of species and niches within the ecosystem. Analyses of ecosystem structure and behavior have traditionally included essential components of the planktonic community such as bacteria, phytoplank-ton, zooplankton, and fish populations.

Table 1 Division of plankton populations in functional size classes

Size (fim) Autotrophs Hetrotrophs

Table 1 Division of plankton populations in functional size classes

Size (fim) Autotrophs Hetrotrophs

Femtoplankton

0.02-0.2

Vira

Picoplankton

0.2-2.0

Picoalgae (bacteria)

Bacteria

Nanoplankton

2-20

Phytoplankton (edible)

Flagellates, ciliates

Microplankton

20-200

Phytoplankton (filamentous)

Ciliates, rotifers

Macroplanktona

200-2000

Phytoplankton (colonial)

Copepods, cladocerans, rotifers

Megaplankton

>2000

Floating macroalgae and plants

Copepods, invertebrate larvae, fish

aSometime referred to as mesoplankton.

aSometime referred to as mesoplankton.

An operational classification of the planktonic populations was originally defined from marine plankton studies and included four groups that have later been extended to six size classes (Table 1). Although such classifications are rigid in terms of natural plasticity, they are useful to describe functionally related organisms. While the size and the productivity of these macrostructures have been natural elements in freshwater and marine water-quality assessment, the microbial food web has not been integrated to the same extent into the analysis of whole ecosystems. The limited knowledge of microbial population dynamics and its relationship to the entire food web structure is given as a reason for this exclusion.

The benthic communities are sometimes seen as more or less detached from the open water processes except for the incontrovertible fact that excess biotic and abiotic material will settle at the bottom of the system over time. However, there is an obvious coupling between the open water and the benthic communities such as organisms that spend their juvenile stages as plankton, but settle as adults at the bottom or on other solid surfaces (e.g., mussels and sea stars) or fish that feed on animals in the sediment, by which they affect their benthic prey populations and are also likely to disturb the physical and chemical conditions - at least temporally - by messing up the sediment surface. Especially in oligtrophic systems a large part of the primary production takes place at the bottom or substrates that are attached to the bottom such as stones and plants. This is obviously the case in many clear-water arctic and alpine lakes where light penetrates deep into the water column and allows growth of photosynthetic organism that in turn can sustain heterotrophs. Thus, there must exist an intensive coupling between the pelagic and the benthic populations. The strength and importance of such linkages are dependent on water depth, altitude, latitude, overall productivity, and species composition.

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