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Determining the effects of nutrient enrichment on macroalgae-dominated reefs : (observational, experimental and predictive capabilities)

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Henríquez Antipa, LA (2016) Determining the effects of nutrient enrichment on macroalgae-dominated reefs : (observational, experimental and predictive capabilities). PhD thesis, University of Tasmania.

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Abstract

The impacts caused by chronic nutrient enrichment on coastal habitats usually
comprise a slow stepwise progression of chemical and biological changes which
can be highly influenced by the physical environment. As a consequence, the
process of change is dependent upon the unique environmental circumstances
in each system and detection of impacts, particularly in the early stages, can be
difficult. In addition, the influence of multiple anthropogenic nutrient sources
interacting at a community or species level further complicates impact
assessments. However, understanding the mechanism by which increasing
changes in nutrient availability might affect the complex nature of the coastal reef
ecology is essential for early detection, subsequent prevention and control of
impacts.
The capacity to predict how natural variability interacts with anthropogenic
stressors is a challenge for both marine scientists and environmental managers.
Natural selective forces in costal habitats (e.g., light levels, wave exposure,
salinity, temperature and species interactions) will influence the degree to which
reef systems respond to sources of nutrients. However, these forces can also
promote the system’s ability to cope with impacts (resistance) or to recover from
a given disturbance (resilience), and will vary spatially and temporally throughout
geographical gradients within the same system. Understanding the system
characteristics and key biological responses may help to determine where
potential impacts may or may not take place.
This study outlines a field experiment, which measurably increased the nutrient
availability in three reef systems to determine changes in macroalgal community
composition, successional stages and classical indicators (i.e., fast-growing
opportunistic species). In addition, this study examined how alternative
indicators, including underlying indicators of impact such as physiological
sensitivity of key macroalgal species), can provide better indication of the status
of impact/ stress. Furthermore, location-specific variations of abiotic factors
were monitored to test if physical drivers, the structuring forces of gradients in
community structure, may improve our understanding of eutrophication.
The results indicate that there was no evidence of major effects of nutrient
enrichment on the overall community structure and that the observed responses
of opportunistic species were not consistent. The abundance of opportunistic
species differed between locations and showed significant effects of small-scale
variability within each community. Physical drivers (variations in wave exposure
and light, as well as differences in salinity and temperature) were correlated with
fundamental differences in the established and the successional community
structure throughout the study region. This suggests that fundamental and
prevailing components of the community (e.g., Caulerpales/Fucales and/or
Encrusting algae/Fucales-dominated reefs) and abiotic fluctuations may
underpin opportunistic algae abundances and define the responses to
enhancement of nutrients. This has important implications for our previous
understanding of nutrient enrichment indicators and how this relationship might
be applied to management (i.e., where the presence of fast-growing
opportunistic species in abundance is indicative of an adverse environmental impact) - as in many cases this interaction may simply represent a difference resulting from prevailing environmental conditions.
The spatial effects of diverse sources of nutrient (i.e. sewage treatment plants,
marine farming, catchment and river inputs) could be differentiated by analysing
the nutrient tissue composition (C, N, P) in key algal species, and was to a lesser
extent evident in their photosynthetic performance, which may require further
research given the limitation that the assessment of Chlorophyll-a fluorescence
may have in macroalgae from nutrient limiting waters. These findings suggest
that the physiological responses of key algal species may constitute a more
reliable indicator of system status. Since some species are likely to reach their
nutrient requirements at low/moderate nutrient concentrations (e.g., fucoids)
they may be good at long-term storage of limiting nutrients that may otherwise
mask species responses to nutrient variation and/ or addition (Pedersen 1995).
For this reason it is also suggested that macroalgae on particular reefs may be
pre-adapted to different nutrient inputs depending on the prevailing nutrient
regime.
In conclusion, these results highlight that in a real life scenario the usefulness of
classical indicators of eutrophication may be constrained by location-specific
gradients and small-scale spatial variability. Spatial patterns in the biophysical
environment (e.g., wave exposure, light environment) indicated gradients in
community structure and habitat attributes that in turn would appear to restrict
eutrophication responses in macroalgal reefs. Inclusion of tissue nutrient levels
and photosynthetic performance of key species in assessments of eutrophication appeared to better define the spatial role of physical drivers (like wave exposure and light), and sources of nutrient availability. Incorporating this
information into ecosystem models may improve the analytical power.
This study provides important data to improve spatial management in systems
with macroalgal reefs and suggests that characterisation of the physical
environment is essential to contextualise the system response and that
evaluation of the sensitivity of key species at a physiological level may provide a
more accurate and effective indication of the impact due to nutrient enrichment
in coastal habitats than the determination of localised increases in opportunistic
species.

Item Type: Thesis (PhD)
Keywords: Eutrophication, Coastal ecosystems, Bioindicators, Coastal Management
Copyright Information:

Copyright 2015 the author

Date Deposited: 05 Oct 2016 23:30
Last Modified: 05 Oct 2016 23:30
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