186
Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
agreement with proposal of Andersen and Hessen (1991), that a change in
zooplankton species composition, from copepods to daphnids, could lead
to a shift from N- to P-limited phytoplankton growth, or vice versa.
While this theory fmds support in a study by Elser et al. (1988), who
found clear evidence for a shift from N- to P-limitation following a change
from copepod to daphnid dominance in the zooplankton community, it
seems likely that such a mechanism can apply only for a restricted range of
N:P loading ratios. If, for lack of better data, we assume that there should
be a relationship between N:P loading ratios and ratios of standing stocks
of total Nand P, we find in the regional eutrophication survey of Faafeng et
al. (1990) that only 3/351 ofthe lakes had total N:total P < 6.3 and 24/ 351
of the lakes had total N:total P < 17.9. In other words, the mechanism of
zooplankton-mediated N- and P-limitation should be limited to about 6%
of the lakes considered in the NIV A study, although the roughness of this
calculation calls for considerable caution.
6.4 The Fate of Zooplankton Egesta: Carbon Cycling and
Chaos
All models considered thus far have implicitly made the simplifying
assumption that algal biomass is the sole food source of zooplankton, and
that transformations of essential mineral nutrients are closely associated
with the dynamics of phyto- and zooplankton populations. It has nevertheless long been recognized that the metabolism of organic carbon in ecosystems can follow two major pathways: a classical grazer food chain and a
parallel detritus food chain (Odum 1962, Odum and de la Cruz 1963). Since
fllter-feeding zooplankton appear to playa major part in both pathways,
the functional distinction between grazer and detritus food chains becomes
blurred in planktonic ecosystems. Most crustacean fllter-feeders have been
shown to both ingest and assimilate appreciable amounts of particulate
detritus (Nauwerck 1963; Wetzel et al. 1972; Langeland and Reinertsen
1982; Hessen et al. 1990), and, at the same time, also produce detritus and
dissolved organic matter from living biomass through egestion of non-assimilated food and through "sloppy" feeding (Lampert 1978; Olsen et al.
1986a; Hessen et al. 1990). Repeated ingestion and egestion by zooplankton
will also gradually transform particulate detritus into forms of dissolved
organic carbon that are more easily available to free-living bacteria.
While the current view holds that planktonic, free-living bacteria derive
much of their energy supply from algal exudates (Riemann and S0ndergaard 1986), a recent compilation of extracellular release rates from different pelagic systems indicates that this carbon source can seldom cover more
than half of the requirements for bacterial growth (Baines and Pace 1991). It
Approaching Planktonic Food Webs: Competition, Coexistence, and Chaos
agreement with proposal of Andersen and Hessen (1991), that a change in
zooplankton species composition, from copepods to daphnids, could lead
to a shift from N- to P-limited phytoplankton growth, or vice versa.
While this theory fmds support in a study by Elser et al. (1988), who
found clear evidence for a shift from N- to P-limitation following a change
from copepod to daphnid dominance in the zooplankton community, it
seems likely that such a mechanism can apply only for a restricted range of
N:P loading ratios. If, for lack of better data, we assume that there should
be a relationship between N:P loading ratios and ratios of standing stocks
of total Nand P, we find in the regional eutrophication survey of Faafeng et
al. (1990) that only 3/351 ofthe lakes had total N:total P < 6.3 and 24/ 351
of the lakes had total N:total P < 17.9. In other words, the mechanism of
zooplankton-mediated N- and P-limitation should be limited to about 6%
of the lakes considered in the NIV A study, although the roughness of this
calculation calls for considerable caution.
6.4 The Fate of Zooplankton Egesta: Carbon Cycling and
Chaos
All models considered thus far have implicitly made the simplifying
assumption that algal biomass is the sole food source of zooplankton, and
that transformations of essential mineral nutrients are closely associated
with the dynamics of phyto- and zooplankton populations. It has nevertheless long been recognized that the metabolism of organic carbon in ecosystems can follow two major pathways: a classical grazer food chain and a
parallel detritus food chain (Odum 1962, Odum and de la Cruz 1963). Since
fllter-feeding zooplankton appear to playa major part in both pathways,
the functional distinction between grazer and detritus food chains becomes
blurred in planktonic ecosystems. Most crustacean fllter-feeders have been
shown to both ingest and assimilate appreciable amounts of particulate
detritus (Nauwerck 1963; Wetzel et al. 1972; Langeland and Reinertsen
1982; Hessen et al. 1990), and, at the same time, also produce detritus and
dissolved organic matter from living biomass through egestion of non-assimilated food and through "sloppy" feeding (Lampert 1978; Olsen et al.
1986a; Hessen et al. 1990). Repeated ingestion and egestion by zooplankton
will also gradually transform particulate detritus into forms of dissolved
organic carbon that are more easily available to free-living bacteria.
While the current view holds that planktonic, free-living bacteria derive
much of their energy supply from algal exudates (Riemann and S0ndergaard 1986), a recent compilation of extracellular release rates from different pelagic systems indicates that this carbon source can seldom cover more
than half of the requirements for bacterial growth (Baines and Pace 1991). It