586
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
nonetheless can help to put into perspective the degree of nutrient enrichment experienced by seagrass
communities in estuarine and nearshore marine environments. Thus, estimated nitrogen yields from the
watersheds of the Chesapeake Bay, USA (23.4 kg
TN ha
−1 year
−1 ; Boynton et al., 1995), Narragansett
Bay, Rhode Island, USA (18.6 kg TN ha
−1 year
−1 ,
Nixon et al., 1995) and Delaware Bay, USA (15.1 kg
TN ha
−1 year
−1 , Nixon et al., 1996) would all appear
to be substantially higher than baseline conditions.
C. Implications for Future Population Growth
As an example drawn from one of several documented cases of recovering ecosystems, seagrass
coverage in Southwest Florida’s estuaries has increased in recent years, mostly as a result of
improvements in water quality that are, in turn, related to massive (and expensive) reductions in point
source nitrogen loads (Kurz et al., 1999; Tomasko,
2002). Seagrass cover in Tampa Bay is estimated
to have increased by approximately 25% between
1982 and 1996, while Sarasota Bay’s cover has increased 19% between 1988 and 1996 (Tomasko,
2002). Charlotte Harbor’s increase has been more
moderate, a 6% increase between 1982 and 1996,
and may be simply natural variability (Tomasko and
Hall, 1999). Lemon Bay’s coverage appears to have
slightly decreased between 1988 and 1996 (Kurz
et al., 1999).
Within the past 20 or so years, estuaries in Southwest Florida have experienced the combination of increased watershed population density and yet nearly
simultaneous improvements in both water quality
and seagrass cover. This situation has been possible
mostly due to three phenomena: (1) estuaries such
as Tampa Bay and Sarasota Bay had been allowed
to significantly degrade by the late 1970s and early
1980s, (2) point sources of pollution (e.g. wastewater treatment plants, industrial discharges) were substantial impacts, and (3) the technology existed to reduce the loads from these point sources. Given this
situation, the public might be forgiven for believing
that future population growth can be accommodated
without further degradation of nearshore waters and
habitats.
However, the possibility exists that the relationship between nitrogen yields and the degree of urbanization of the watershed evident when comparing estuaries against each other also holds true when
comparing single systems over time. That is, it is possible that comparing different systems against each
other is a useful ‘space for time’ substitution to predict what might happen for a single ecosystem over
the years. If that is true, then recent increases in
seagrass coverage in Southwest Florida’s estuaries
(or any other locations that follow a similar pattern)
might potentially be offset in the future by increased
nitrogen loads associated with continued population
growth in their watersheds.
IV. Recommended Research Directions
Part of the process of reviewing the current knowledge of impacts of eutrophication, sediments and
contaminants on seagrasses, is to identify gaps in
our understanding and recommend future research
directions. In the following paragraphs we have described a series of topics where we feel research effort would be best applied.
The physicochemical conditions established with
a laboratory-based exposure chamber (aquarium or
mesocosm) do not replicate the in situ ecosystem.
Clearly, field based assessments will be closer to reality and generally may show increased tolerance
to pollutants. This increased tolerance is not linked
to the plants per se; it is more likely the capacity of the undisturbed ecosystem to ameliorate the
toxicants. For example, the microbial assemblage
in a laboratory-based experiment bears limited resemblance to the complex system supported in situ.
The interaction of sediment, microbial, fauna activities, and water quality influences on availability of
toxicants is not replicated in laboratory trials. It is
clearly recognized that little understanding exists to
explain the complex process involved with field conditions, in comparison to the relative wealth of information derived from controlled laboratory investigations.
Assessing pollutant impacts in a toxicological approach will provide a better understanding of the
risks posed by pollution to seagrass survival. Dose
response is a complex interaction of concentration
and exposure period; therefore short-term high dose
experiments do not provide a true indication of the
plant’s tolerance to these contaminants. It is important that dose-response experiments are performed
on a range of contaminants that are environmentally
relevant. It is relatively easy to expose a plant with
a toxic dose of pollutant, but if the endpoint is not
P. J. Ralph, D. Tomasko, K. Moore, S. Seddon, and C. M. O. Macinnis-Ng
nonetheless can help to put into perspective the degree of nutrient enrichment experienced by seagrass
communities in estuarine and nearshore marine environments. Thus, estimated nitrogen yields from the
watersheds of the Chesapeake Bay, USA (23.4 kg
TN ha
−1 year
−1 ; Boynton et al., 1995), Narragansett
Bay, Rhode Island, USA (18.6 kg TN ha
−1 year
−1 ,
Nixon et al., 1995) and Delaware Bay, USA (15.1 kg
TN ha
−1 year
−1 , Nixon et al., 1996) would all appear
to be substantially higher than baseline conditions.
C. Implications for Future Population Growth
As an example drawn from one of several documented cases of recovering ecosystems, seagrass
coverage in Southwest Florida’s estuaries has increased in recent years, mostly as a result of
improvements in water quality that are, in turn, related to massive (and expensive) reductions in point
source nitrogen loads (Kurz et al., 1999; Tomasko,
2002). Seagrass cover in Tampa Bay is estimated
to have increased by approximately 25% between
1982 and 1996, while Sarasota Bay’s cover has increased 19% between 1988 and 1996 (Tomasko,
2002). Charlotte Harbor’s increase has been more
moderate, a 6% increase between 1982 and 1996,
and may be simply natural variability (Tomasko and
Hall, 1999). Lemon Bay’s coverage appears to have
slightly decreased between 1988 and 1996 (Kurz
et al., 1999).
Within the past 20 or so years, estuaries in Southwest Florida have experienced the combination of increased watershed population density and yet nearly
simultaneous improvements in both water quality
and seagrass cover. This situation has been possible
mostly due to three phenomena: (1) estuaries such
as Tampa Bay and Sarasota Bay had been allowed
to significantly degrade by the late 1970s and early
1980s, (2) point sources of pollution (e.g. wastewater treatment plants, industrial discharges) were substantial impacts, and (3) the technology existed to reduce the loads from these point sources. Given this
situation, the public might be forgiven for believing
that future population growth can be accommodated
without further degradation of nearshore waters and
habitats.
However, the possibility exists that the relationship between nitrogen yields and the degree of urbanization of the watershed evident when comparing estuaries against each other also holds true when
comparing single systems over time. That is, it is possible that comparing different systems against each
other is a useful ‘space for time’ substitution to predict what might happen for a single ecosystem over
the years. If that is true, then recent increases in
seagrass coverage in Southwest Florida’s estuaries
(or any other locations that follow a similar pattern)
might potentially be offset in the future by increased
nitrogen loads associated with continued population
growth in their watersheds.
IV. Recommended Research Directions
Part of the process of reviewing the current knowledge of impacts of eutrophication, sediments and
contaminants on seagrasses, is to identify gaps in
our understanding and recommend future research
directions. In the following paragraphs we have described a series of topics where we feel research effort would be best applied.
The physicochemical conditions established with
a laboratory-based exposure chamber (aquarium or
mesocosm) do not replicate the in situ ecosystem.
Clearly, field based assessments will be closer to reality and generally may show increased tolerance
to pollutants. This increased tolerance is not linked
to the plants per se; it is more likely the capacity of the undisturbed ecosystem to ameliorate the
toxicants. For example, the microbial assemblage
in a laboratory-based experiment bears limited resemblance to the complex system supported in situ.
The interaction of sediment, microbial, fauna activities, and water quality influences on availability of
toxicants is not replicated in laboratory trials. It is
clearly recognized that little understanding exists to
explain the complex process involved with field conditions, in comparison to the relative wealth of information derived from controlled laboratory investigations.
Assessing pollutant impacts in a toxicological approach will provide a better understanding of the
risks posed by pollution to seagrass survival. Dose
response is a complex interaction of concentration
and exposure period; therefore short-term high dose
experiments do not provide a true indication of the
plant’s tolerance to these contaminants. It is important that dose-response experiments are performed
on a range of contaminants that are environmentally
relevant. It is relatively easy to expose a plant with
a toxic dose of pollutant, but if the endpoint is not