74
K. E. Havens
following sections: a description of the plankton food web;
an evaluation of data regarding how biomass distribution
among components of the plankton food web changes along
a gradient from nutrient-poor to nutrient-rich lakes; a consideration of how changes in the structure of plankton food
webs affects their function in regard to their ability to effectively transfer energy to fish and other biota at higher trophic
levels; a brief discussion of how other changes in the lake
ecosystem influence how the plankton food web responds
to eutrophication; and implications for the management of
culturally eutrophic lakes.
6.2 The Plankton Food Web
Limnologists’ view of the plankton food web has changed
considerably since the mid-1900s when it was considered
as a chain where phytoplankton fixed carbon that was then
transferred directly to zooplankton and then to fish and other
predators. This traditional view was supplemented by the
concept of additional microbial pathways in the 1980s (Azam
et al. 1983) and it is now widely recognized that bacteria
and protozoa also play an important role in energy transfer
and nutrient cycling in the plankton. Five major components
and nine major links characterize the contemporary model
of the plankton food web (Fig. 6.1). Phytoplankton fix carbon by photosynthesis and are consumed by a variety of different types of protozoa (including ciliates and flagellates)
by microzooplankton such as rotifers and immature stages
(nauplii) of copepods, and by macrozooplankton (cladocerans and copepods). A second route of energy flow occurs
from protozoa to microzooplankton and macrozooplankton.
Bacteria metabolize organic carbon that enters the lake from
outside sources as well as carbon excreted into the water by
phytoplankton, protozoa, and zooplankton. Bacteria are a
food source for protozoa, microzooplankton, and macrozooplankton.
The Dashed arrows in Fig. 6.1 roughly correspond to
energy flows in the traditionally recognized grazing food
chain links in the web and the solid arrows to the microbial
food web links. Although quantitative evidence regarding
the importance of microbial links emerged in the 1980s
(Azam et al. 1983; Sherr et al. 1987; Porter et al. 1988),
their importance was conceptually identified by Lindeman (1942) who developed an energy flow diagram that
included bacteria and links to and from phytoplankton,
zooplankton, and dissolved materials. Gliwicz (1969) first
speculated that the importance of microbial links may increase with eutrophication, a prediction that was proven
to be correct based on results of later studies (Munawar
et al. 2011).
6.3 The Structure of Plankton Food Webs
Versus Eutrophication
One typical and highly visible symptom of cultural eutrophication is increased biomass and dominance of large inedible
cyanobacteria (e.g., Smith et al. 2006). Coincidentally, there
are changes in the biomass and composition of zooplankton—including a loss of large species of Daphnia, and increased dominance of small rotifers and copepods (Weisse
and Stockner 1992; Havens et al. 1996). Auer et al. (2004)
performed a comprehensive analysis of plankton food web
structure along a eutrophication gradient, comparing plankton communities in 55 lakes in northern Germany that ranged
from mesotrophic to hypereutrophic. All the components of
the food web displayed an increase in their biomass with
greater levels of enrichment, indexed by chlorophyll concentration, although the rates of increase varied (Fig. 6.2).
As a result, the proportional biomass changed: mesotrophic
lakes had about 30 % of the plankton biomass in zooplankton
and about 45 % in phytoplankton (Fig. 6.3a). Lesser amounts
of biomass occurred in bacteria, heterotrophic flagellates,
and ciliates. Over the gradient of enrichment, relative biomass of zooplankton dropped, to a low of around 15 % in
WZKdKK
D/ZKͲKKW>E
W,zdKW>E
traditional grazing food chain and dashed arrows correspond to links in
the microbial food web. Numerous species comprise each of the components shown here—individual species are shown as examples only
K. E. Havens
following sections: a description of the plankton food web;
an evaluation of data regarding how biomass distribution
among components of the plankton food web changes along
a gradient from nutrient-poor to nutrient-rich lakes; a consideration of how changes in the structure of plankton food
webs affects their function in regard to their ability to effectively transfer energy to fish and other biota at higher trophic
levels; a brief discussion of how other changes in the lake
ecosystem influence how the plankton food web responds
to eutrophication; and implications for the management of
culturally eutrophic lakes.
6.2 The Plankton Food Web
Limnologists’ view of the plankton food web has changed
considerably since the mid-1900s when it was considered
as a chain where phytoplankton fixed carbon that was then
transferred directly to zooplankton and then to fish and other
predators. This traditional view was supplemented by the
concept of additional microbial pathways in the 1980s (Azam
et al. 1983) and it is now widely recognized that bacteria
and protozoa also play an important role in energy transfer
and nutrient cycling in the plankton. Five major components
and nine major links characterize the contemporary model
of the plankton food web (Fig. 6.1). Phytoplankton fix carbon by photosynthesis and are consumed by a variety of different types of protozoa (including ciliates and flagellates)
by microzooplankton such as rotifers and immature stages
(nauplii) of copepods, and by macrozooplankton (cladocerans and copepods). A second route of energy flow occurs
from protozoa to microzooplankton and macrozooplankton.
Bacteria metabolize organic carbon that enters the lake from
outside sources as well as carbon excreted into the water by
phytoplankton, protozoa, and zooplankton. Bacteria are a
food source for protozoa, microzooplankton, and macrozooplankton.
The Dashed arrows in Fig. 6.1 roughly correspond to
energy flows in the traditionally recognized grazing food
chain links in the web and the solid arrows to the microbial
food web links. Although quantitative evidence regarding
the importance of microbial links emerged in the 1980s
(Azam et al. 1983; Sherr et al. 1987; Porter et al. 1988),
their importance was conceptually identified by Lindeman (1942) who developed an energy flow diagram that
included bacteria and links to and from phytoplankton,
zooplankton, and dissolved materials. Gliwicz (1969) first
speculated that the importance of microbial links may increase with eutrophication, a prediction that was proven
to be correct based on results of later studies (Munawar
et al. 2011).
6.3 The Structure of Plankton Food Webs
Versus Eutrophication
One typical and highly visible symptom of cultural eutrophication is increased biomass and dominance of large inedible
cyanobacteria (e.g., Smith et al. 2006). Coincidentally, there
are changes in the biomass and composition of zooplankton—including a loss of large species of Daphnia, and increased dominance of small rotifers and copepods (Weisse
and Stockner 1992; Havens et al. 1996). Auer et al. (2004)
performed a comprehensive analysis of plankton food web
structure along a eutrophication gradient, comparing plankton communities in 55 lakes in northern Germany that ranged
from mesotrophic to hypereutrophic. All the components of
the food web displayed an increase in their biomass with
greater levels of enrichment, indexed by chlorophyll concentration, although the rates of increase varied (Fig. 6.2).
As a result, the proportional biomass changed: mesotrophic
lakes had about 30 % of the plankton biomass in zooplankton
and about 45 % in phytoplankton (Fig. 6.3a). Lesser amounts
of biomass occurred in bacteria, heterotrophic flagellates,
and ciliates. Over the gradient of enrichment, relative biomass of zooplankton dropped, to a low of around 15 % in
WZKdKK
D/ZKͲKKW>E
W,zdKW>E
traditional grazing food chain and dashed arrows correspond to links in
the microbial food web. Numerous species comprise each of the components shown here—individual species are shown as examples only