78
K. E. Havens
and other large zooplankton against plankton-eating fish, or
refuge for those fish against larger fish predators. Depending on the role that the plants were playing in the particular
lake, loss of SAV could lead to a reduction or increase in the
zooplankton. It should be noted that there is some debate as
to whether the rapid switch from clear to turbid is typical or
unusual. Sayer et al. (2010), for example, studied SAV and
phytoplankton dynamics in 39 shallow lakes in the UK and
Denmark over a broad nutrient gradient. They documented
a long-term loss of SAV in many lakes that were affected by
cultural eutrophication and suggested that there was a gradual replacement of plants by phytoplankton over a time period
of 10 to 100 years, rather than the sudden switch predicted
by the alternative states model. Other recent studies support
this view (e.g., James et al. 2005). Yet, there are cases where
transitions happen more quickly. In Lake Okeechobee, Florida, for example, we observed a switch from turbid water
with dense cyanobacteria blooms to clear water with widespread SAV ( Chara) in just 1 month (Havens et al. 2001).
To a large extent, the rate of change may depend on the bottom morphology. In the case of Lake Okeechobee, the switch
happened across a large area of shallow water with a nearly
flat bottom versus the gradually sloping shoreline that is
common in many lakes.
6.6 Changes in Fish and Effects on Plankton
Changes in fish assemblages also occur with eutrophication and this may be owing, in part, to the afore-mentioned
changes in the plankton food web; similarly, fish may have
strong top-down impacts on the plankton. In particular, shallow eutrophic lakes support high densities of planktivorous
and omnivorous fish (Jeppesen et al. 2007) including species
that are voracious sight-feeding predators of zooplankton, as
well as filter-feeding omnivores such as gizzard shad that
consume benthos, phytoplankton, and zooplankton (Crisman and Beaver 1990). Warm-water lakes in the subtropics
appear to have particularly high predation pressure on the
zooplankton and have extreme low ratios of zooplankton to
phytoplankton biomass, absence of Daphnia (e.g., Jeppesen
et al. 2007; Havens et al. 2009), and lack of control of the
phytoplankton by zooplankton grazers (Havens et al. 1996).
In these lakes, most energy flow happens in microbial pathways (Crisman and Beaver 1990; Work et al. 2005).
Although some (e.g., Hart 2011) suggest that large inedible phytoplankton are responsible for these characteristics,
most research indicates that the paucity of zooplankton is a
direct response to fish predation (Meerhoff et al. 2007; Iglesias et al. 2008; Jeppesen et al. 2007; Havens and Beaver
2012). Jeppesen et al. (2000a) documented that in New Zealand lakes, there is a strong positive correlation between fish
and phytoplankton biomass, but no such correlation between
zooplankton and phytoplankton. These results indicate topdown control of the zooplankton by fish predation. Jeppesen
et al. (2003) also examined data from 466 temperate to arctic lakes and found a strong pattern: fish control over large
zooplankton is strongest in oligotrophic and highly eutrophic
lakes and weakest in mesotrophic lakes (consistent with the
findings about unimodal relationships discussed earlier).
Jeppesen et al. (2000b) found that across a TP gradient of < 50
to > 400 g l
−1
, there was a decline in piscivores, an increase
in cyprinids, a shift to smaller zooplankton, and a reduction
in mean body weight of cladocerans from 5.1 to 1.5 g. The
ratio of zooplankton to phytoplankton biomass dropped from
0.5 to 0.1 as biomass of phytoplankton increased 15-fold. In
three lakes where fish kills occurred, the trends in plankton
size and biomass quickly reversed, similar to the findings
noted earlier in lakes that underwent reductions in external
nutrient loads. Similarly, Auer et al. (2004) found that increases in rotifers, ciliates, and nanoflagellates seen in hypereutrophic lakes occur because of relaxed grazing pressure
by larger Cladocera, which have been eliminated by fish. In
Lake Okechobee, Florida, we recently found that seasonal
changes in the ratio of zooplankton to phytoplankton biomass are almost entirely driven by seasonal changes in biomass of the zooplankton and that those changes coincide
with periods of high versus reduced fish predation.
6.7 Implications for Lake Management
Reduction of external nutrient loads is a prerequisite for reversing cultural eutrophication. At the same time, the process
may be hastened if it is possible to perform some ecological
engineering of the food web. This sometimes is done by manipulating the density of planktivorous or omnivorous fish in
order to reduce grazing pressure on Daphnia, so that animal
can become more abundant and graze down the phytoplankton (e.g., Shapiro and Wright 1984; Carpenter et al. 1987).
The goal is to reduce the amount of phytoplankton per unit
of P in the lake, via an animal that can filter large volumes of
water and feed on nearly the entire plankton food web, from
phytoplankton to bacteria. The challenge is maintaining the
lowered biomass of planktivorous and omnivorous fish.
At some point in the rehabilitation process, where the lake
reestablishes an oxygenated deep water zone in summer, it
may be possible to introduce piscivorous fish to naturally
keep planktivores under control. These approaches are not
as effective in the subtropics, where large Daphnia generally
do not occur, and where the microbial food web is more
important than in temperate lakes of a similar degree of enrichment (Crisman and Beaver 1990; Havens et al. 2007).
Thus, in the subtropics, control of external nutrient loading
K. E. Havens
and other large zooplankton against plankton-eating fish, or
refuge for those fish against larger fish predators. Depending on the role that the plants were playing in the particular
lake, loss of SAV could lead to a reduction or increase in the
zooplankton. It should be noted that there is some debate as
to whether the rapid switch from clear to turbid is typical or
unusual. Sayer et al. (2010), for example, studied SAV and
phytoplankton dynamics in 39 shallow lakes in the UK and
Denmark over a broad nutrient gradient. They documented
a long-term loss of SAV in many lakes that were affected by
cultural eutrophication and suggested that there was a gradual replacement of plants by phytoplankton over a time period
of 10 to 100 years, rather than the sudden switch predicted
by the alternative states model. Other recent studies support
this view (e.g., James et al. 2005). Yet, there are cases where
transitions happen more quickly. In Lake Okeechobee, Florida, for example, we observed a switch from turbid water
with dense cyanobacteria blooms to clear water with widespread SAV ( Chara) in just 1 month (Havens et al. 2001).
To a large extent, the rate of change may depend on the bottom morphology. In the case of Lake Okeechobee, the switch
happened across a large area of shallow water with a nearly
flat bottom versus the gradually sloping shoreline that is
common in many lakes.
6.6 Changes in Fish and Effects on Plankton
Changes in fish assemblages also occur with eutrophication and this may be owing, in part, to the afore-mentioned
changes in the plankton food web; similarly, fish may have
strong top-down impacts on the plankton. In particular, shallow eutrophic lakes support high densities of planktivorous
and omnivorous fish (Jeppesen et al. 2007) including species
that are voracious sight-feeding predators of zooplankton, as
well as filter-feeding omnivores such as gizzard shad that
consume benthos, phytoplankton, and zooplankton (Crisman and Beaver 1990). Warm-water lakes in the subtropics
appear to have particularly high predation pressure on the
zooplankton and have extreme low ratios of zooplankton to
phytoplankton biomass, absence of Daphnia (e.g., Jeppesen
et al. 2007; Havens et al. 2009), and lack of control of the
phytoplankton by zooplankton grazers (Havens et al. 1996).
In these lakes, most energy flow happens in microbial pathways (Crisman and Beaver 1990; Work et al. 2005).
Although some (e.g., Hart 2011) suggest that large inedible phytoplankton are responsible for these characteristics,
most research indicates that the paucity of zooplankton is a
direct response to fish predation (Meerhoff et al. 2007; Iglesias et al. 2008; Jeppesen et al. 2007; Havens and Beaver
2012). Jeppesen et al. (2000a) documented that in New Zealand lakes, there is a strong positive correlation between fish
and phytoplankton biomass, but no such correlation between
zooplankton and phytoplankton. These results indicate topdown control of the zooplankton by fish predation. Jeppesen
et al. (2003) also examined data from 466 temperate to arctic lakes and found a strong pattern: fish control over large
zooplankton is strongest in oligotrophic and highly eutrophic
lakes and weakest in mesotrophic lakes (consistent with the
findings about unimodal relationships discussed earlier).
Jeppesen et al. (2000b) found that across a TP gradient of < 50
to > 400 g l
−1
, there was a decline in piscivores, an increase
in cyprinids, a shift to smaller zooplankton, and a reduction
in mean body weight of cladocerans from 5.1 to 1.5 g. The
ratio of zooplankton to phytoplankton biomass dropped from
0.5 to 0.1 as biomass of phytoplankton increased 15-fold. In
three lakes where fish kills occurred, the trends in plankton
size and biomass quickly reversed, similar to the findings
noted earlier in lakes that underwent reductions in external
nutrient loads. Similarly, Auer et al. (2004) found that increases in rotifers, ciliates, and nanoflagellates seen in hypereutrophic lakes occur because of relaxed grazing pressure
by larger Cladocera, which have been eliminated by fish. In
Lake Okechobee, Florida, we recently found that seasonal
changes in the ratio of zooplankton to phytoplankton biomass are almost entirely driven by seasonal changes in biomass of the zooplankton and that those changes coincide
with periods of high versus reduced fish predation.
6.7 Implications for Lake Management
Reduction of external nutrient loads is a prerequisite for reversing cultural eutrophication. At the same time, the process
may be hastened if it is possible to perform some ecological
engineering of the food web. This sometimes is done by manipulating the density of planktivorous or omnivorous fish in
order to reduce grazing pressure on Daphnia, so that animal
can become more abundant and graze down the phytoplankton (e.g., Shapiro and Wright 1984; Carpenter et al. 1987).
The goal is to reduce the amount of phytoplankton per unit
of P in the lake, via an animal that can filter large volumes of
water and feed on nearly the entire plankton food web, from
phytoplankton to bacteria. The challenge is maintaining the
lowered biomass of planktivorous and omnivorous fish.
At some point in the rehabilitation process, where the lake
reestablishes an oxygenated deep water zone in summer, it
may be possible to introduce piscivorous fish to naturally
keep planktivores under control. These approaches are not
as effective in the subtropics, where large Daphnia generally
do not occur, and where the microbial food web is more
important than in temperate lakes of a similar degree of enrichment (Crisman and Beaver 1990; Havens et al. 2007).
Thus, in the subtropics, control of external nutrient loading