77
6 Lake Eutrophication and Plankton Food Webs
to its quantity of grazing and wide size range of grazed particles. In hypereutrophic lakes, size again interferes with
direct energy transfer in the grazing chain. Large cyanobacteria such as Anabaena coexist with small zooplankton such
as Bosmina that cannot eat those phytoplankton (Fig. 6.6b).
The cyanobacteria excrete carbon that is taken up by bacteria and then is routed up through a microbial web to the
zooplankton.
Quantitative data documenting the unimodal pattern in
food web efficiency were presented by Jeppesen et al. (2003)
based on a comprehensive survey of 466 lakes spanning the
temperate to arctic zones and encompassing a broad range of
trophic states. Other studies documenting the predominance
of a microbial food web in eutrophic lakes include Riemann
and Sondergaard (1986), who compared the plankton in two
lakes differing in degree of nutrient enrichment, and Cole
et al. (1988), who compared the plankton in 24 lakes across
an eutrophication gradient.
Because complex food webs lead to greater energy loss
compared with one-step chains where Daphnia directly
graze phytoplankton, eutrophication-induced changes in
food web efficiency have implications for higher trophic levels. At some point, the upward flow of energy may become
insufficient to support an additional trophic level, such as
piscivorous fish (Persson et al. 1988). Havens et al. (2000)
documented the low ecological transfer efficiency that is
typical of highly eutrophic lakes. They measured transfer
efficiency of plankton when cyanobacteria dominated the
phytoplankton and copepods dominated the zooplankton
using a radiotracer method developed by Ducklow et al.
(1986). Havens et al. (2000) inoculated whole water samples
including indigenous plankton with
14
C-labeled glucose or
bicarbonate (carbon sources for bacteria and phytoplankton,
respectively). In both cases, they found that just 0.1 % to
1.0 % of the carbon was transferred to >200 m size particles
(macrozooplankton) after 4 h of incubation. The same results
were obtained when the experiment was repeated four more
times over a year-long period. Similarly, when Sommaruga
(1995) developed a carbon budget for a eutrophic lake in
Uruguay dominated by filamentous cyanobacteria and small
zooplankton, they measured extreme low ecological transfer efficiencies. This situation may explain, in part, why it
is common to find hypereutrophic lakes dominated by fish,
such as gizzard shad and carp, which can feed low in the
food web, i.e., graze directly on phytoplankton and detritus
(Bays and Crisman 1983; Allen et al. 2000), and thereby circumvent the inefficient plankton food web.
6.5 Changes in Plants and Effects on Plankton
Other profound changes, described in this book, occur in
lakes as they undergo eutrophication, and some of them can
affect the plankton food web. One notable change is the loss
of submerged aquatic vegetation (SAV) when biomass of cyanobacteria becomes extremely high in the phytoplankton.
Cyanobacteria blooms block light penetration to the extent
that they shade out the SAV. The phenomenon is most common in shallow eutrophic lakes and it has been suggested
that it can happen quickly, with a lake quickly transitioning
from a clear water phase with abundant SAV and low phytoplankton biomass and a turbid water phase with little or no
SAV and high phytoplankton biomass as the lake crosses a
nutrient threshold (Scheffer et al. 1993). When SAV is lost
from a lake, so too is the refuge they provide for Daphnia
Daphnia
Flagellates
DOC
BACT
CO 2
Bosmina
Anabaena
DOC
BACT
CO 2
a
b
Fig. 6.6 Simple diagrams of C flow in food webs contrasting dominance of Daphnia and edible flagellates (a) with dominance by Bosmina and inedible cyanobacteria (b). The thickness of arrows corresponds to relative amounts of C flow
PHYT
(600)
HFL
(60)
PFL
(80)
BAC
(70)
CIL
(70)
MIC
(2)
MAC
(30)
Metazoan consumer
Protozoan consumer
Primary producer
Decomposer
Size of boxes correspond to
Carbon biomass (mg C L -1 )
And arrows indicate trophic
Links identified by Work et al.
(2005)
Fig. 6.5 Partitioning of carbon among various components of the
plankton food web of a subtropical eutrophic lake, Lake Okeechobee,
Florida. The data are based on Havens et al. (2007). MAC macrozooplankton, MIC microzooplankton, CIL ciliates, HFL heterotrophic
flagellates, PFL phototrophic flagellates, BAC bacteria, PHYT phytoplankton
6 Lake Eutrophication and Plankton Food Webs
to its quantity of grazing and wide size range of grazed particles. In hypereutrophic lakes, size again interferes with
direct energy transfer in the grazing chain. Large cyanobacteria such as Anabaena coexist with small zooplankton such
as Bosmina that cannot eat those phytoplankton (Fig. 6.6b).
The cyanobacteria excrete carbon that is taken up by bacteria and then is routed up through a microbial web to the
zooplankton.
Quantitative data documenting the unimodal pattern in
food web efficiency were presented by Jeppesen et al. (2003)
based on a comprehensive survey of 466 lakes spanning the
temperate to arctic zones and encompassing a broad range of
trophic states. Other studies documenting the predominance
of a microbial food web in eutrophic lakes include Riemann
and Sondergaard (1986), who compared the plankton in two
lakes differing in degree of nutrient enrichment, and Cole
et al. (1988), who compared the plankton in 24 lakes across
an eutrophication gradient.
Because complex food webs lead to greater energy loss
compared with one-step chains where Daphnia directly
graze phytoplankton, eutrophication-induced changes in
food web efficiency have implications for higher trophic levels. At some point, the upward flow of energy may become
insufficient to support an additional trophic level, such as
piscivorous fish (Persson et al. 1988). Havens et al. (2000)
documented the low ecological transfer efficiency that is
typical of highly eutrophic lakes. They measured transfer
efficiency of plankton when cyanobacteria dominated the
phytoplankton and copepods dominated the zooplankton
using a radiotracer method developed by Ducklow et al.
(1986). Havens et al. (2000) inoculated whole water samples
including indigenous plankton with
14
C-labeled glucose or
bicarbonate (carbon sources for bacteria and phytoplankton,
respectively). In both cases, they found that just 0.1 % to
1.0 % of the carbon was transferred to >200 m size particles
(macrozooplankton) after 4 h of incubation. The same results
were obtained when the experiment was repeated four more
times over a year-long period. Similarly, when Sommaruga
(1995) developed a carbon budget for a eutrophic lake in
Uruguay dominated by filamentous cyanobacteria and small
zooplankton, they measured extreme low ecological transfer efficiencies. This situation may explain, in part, why it
is common to find hypereutrophic lakes dominated by fish,
such as gizzard shad and carp, which can feed low in the
food web, i.e., graze directly on phytoplankton and detritus
(Bays and Crisman 1983; Allen et al. 2000), and thereby circumvent the inefficient plankton food web.
6.5 Changes in Plants and Effects on Plankton
Other profound changes, described in this book, occur in
lakes as they undergo eutrophication, and some of them can
affect the plankton food web. One notable change is the loss
of submerged aquatic vegetation (SAV) when biomass of cyanobacteria becomes extremely high in the phytoplankton.
Cyanobacteria blooms block light penetration to the extent
that they shade out the SAV. The phenomenon is most common in shallow eutrophic lakes and it has been suggested
that it can happen quickly, with a lake quickly transitioning
from a clear water phase with abundant SAV and low phytoplankton biomass and a turbid water phase with little or no
SAV and high phytoplankton biomass as the lake crosses a
nutrient threshold (Scheffer et al. 1993). When SAV is lost
from a lake, so too is the refuge they provide for Daphnia
Daphnia
Flagellates
DOC
BACT
CO 2
Bosmina
Anabaena
DOC
BACT
CO 2
a
b
Fig. 6.6 Simple diagrams of C flow in food webs contrasting dominance of Daphnia and edible flagellates (a) with dominance by Bosmina and inedible cyanobacteria (b). The thickness of arrows corresponds to relative amounts of C flow
PHYT
(600)
HFL
(60)
PFL
(80)
BAC
(70)
CIL
(70)
MIC
(2)
MAC
(30)
Metazoan consumer
Protozoan consumer
Primary producer
Decomposer
Size of boxes correspond to
Carbon biomass (mg C L -1 )
And arrows indicate trophic
Links identified by Work et al.
(2005)
Fig. 6.5 Partitioning of carbon among various components of the
plankton food web of a subtropical eutrophic lake, Lake Okeechobee,
Florida. The data are based on Havens et al. (2007). MAC macrozooplankton, MIC microzooplankton, CIL ciliates, HFL heterotrophic
flagellates, PFL phototrophic flagellates, BAC bacteria, PHYT phytoplankton