Abstract
In this chapter, we examine the production, use, and loss of dissolved and particulate organic matter in marine food webs. We first highlight microbial aspects of food webs, a subject that has recently undergone major advances. We then review past and current views of marine food webs, briefly mention some aspects of dissolved organic matter, then take a closer look at the major groups of taxa involved in the dynamics of dissolved organic matter, go on with a discussion of a series of loose to obligate interspecies relationships, and then end with an introduction to consumers of particulate food items.
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Notes
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Kirchman (2012) briefly reviewed all these methods.
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Comparative studies such as Fig. 6-7 provide useful views across a wide range of systems, but the interpretation of cross-system data should be conditioned by certain caveats. First, cross-system comparisons unavoidably confound effects of variables of interest (bacteria and phytoplankton abundance, in the case of Fig. 6-7) with effects of other variables inherent in comparisons made across bacterial abundance and activity (White et al. 1991); in cold waters, for example, bacteria are less effective at using organic substrates for growth (Wiebe et al. 1992). The second caveat is that conclusions from cross-site comparisons apply to the entire data set, and may not apply to any one place in particular. For example, while for the aggregate data from all marine sites in Fig. 6-7 there is a positive relation between phytoplankton and bacteria, we should not assume that the same holds for a given parcel of water. Third, correlations do not necessarily identify causes; it is difficult to be sure that phytoplankton is the governing agent in the relationship being studied. Manipulative experiments that include or exclude factors to be tested are better suited for study of how specific communities function. Manipulations do have drawbacks: they may be difficult to carry out, usually can only be applied to small-spatial scales, and sometimes create unrealistic conditions. Nonetheless, if well conceived, manipulations have the advantages of establishing causality and testing specific questions unconfounded by nuisance variables. The ideal situation is to establish mechanisms and causes by experimentation, and to extend the range of application by cross-system comparisons.
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There are many nanoflagellates that are both heterotrophs and autotrophs (Estep et al. 1986). These species contain chlorophyll and can also engulf particulate prey.
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Thacker and Freeman (2012) reviewed the results of shading experiments aimed at determining the relative importance of nutritional contributions by symbionts relative to heterotrophic feeding by sponges. Some sponge species had obligate dependency on symbiont production, and fared poorly in the shade. Other sponges showed no effect of shading, so that the microbes probably simply used the sponges as providers of a place to live. Other species of sponges to some degree depended on the organic subsidies provided by the cyanobacterial symbionts, but managed to increase heterotrophic feeding on particles, which allowed the host to grow even in the shade, therefore displaying a facultative mutualism.
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In terrestrial plants extensive support and vascular systems are needed to keep photosynthetic tissues exposed to adequate light and transport nutrients; in the water column, water movement supplies these functions. The extensive, nongrowing tissues of terrestrial plants slow turnover rates (Table 2-3). This contrast is also important in chemical defense, since many of the deterrents are usually chemically associated with structural tissues.
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The relationship between nutrients and classic phyto- and zooplankton has been simulated in many so-called Nutrient-Phytoplankton-Zooplankton (NPZ) models (Riley 1947, 1963; Steele 1974a, b; Wroblewski and O’Brien 1976; Landry 1977; Davis 1982; among others). Newer models introduce size-based dynamics (Moloney et al. 1991), reflecting new knowledge about food webs, and add trophic levels (nutrient-plankton-fish; Daewel et al. 2014). Turner et al. (2014) provide comparisons of different versions of NPZ models. Although progress has been made, there is still uncertainty about the formulation of the links among these components of food webs.
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Valiela, I. (2015). Use and Transfer of Organic Matter in Marine Food Webs. In: Marine Ecological Processes. Springer, New York, NY. https://doi.org/10.1007/978-0-387-79070-1_6
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DOI: https://doi.org/10.1007/978-0-387-79070-1_6
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