How to Use
The construction and interpretation of species response curves is specialists work. It requires ecological laboratory and field work experience and scientific analytical skills. Even the use of a database of species response curves requires additional site specific monitoring of relevant environmental parameters site specific ecological interpretation. Depending on the quality and accuracy of the predicted environmental impact, and its duration, of infrastructural developments, species response curves can be a very powerful tool in the hands of coastal managers, planners and constructors.
- Experiments
- Response Trajectories
- Iterative process
1. Experiments
In order to determine the vulnerability of seagrasses and seagrass ecosystems to the effects of increased suspended sediment levels (e.g. light reduction and increased sedimentation) laboratory/mesocosm and field experiments and field monitoring programmes may be set-up. These experiments will give insight into different response mechanisms and associated response timescales in relation to stress intensity and stress duration, and the stress resilience of seagrasses, seagrass communities and ecosystems.
Laboratory and mesocosm experiments
Laboratory and mesocosm experiments are especially useful to determine the physiological and morphological responses of a single species to stress under controlled or semi-controlled circumstances. Herewith, by manipulating the stress intensity and duration, the potential specific tolerance limits of a species to a stress factor and threshold values for lethal effects may be determined. The disadvantage is that the plants are taken from their natural environment and placed in an artificial environment which may in itself be a stressful situation. It is also often unclear how plants respond to this handling and to their new environment and to distinguish the effects thereof from the effects of the stress treatment itself.
Field experiments
Field experiments are especially useful to investigate the response of more than one species simultaneously and in their natural environment. These experiments take account of the natural (competitive) ecosystem interactions between different (seagrass) species and individuals and their environment. The resulting tolerance limits of species to a stress factor and threshold values for lethal effects represent more relevant values than the potential tolerance limits determined in lab experiments. Furthermore, these experiments also give insight in responses to stress on ecosystem and community level. The disadvantage is that variations in environmental conditions (e.g. temperature) can often not be controlled and that changes in ecosystem interactions relevant for the interpretation of the results are difficult to predict and to take into consideration beforehand. Furthermore, differences in specific tolerance limits between species may cause one species to respond negatively to the stress factor, and other species to take advantage of the decline in fitness of the more sensitive species and therefore profit from stress, although this may not be the case in a lab experiment. The presence of the one species, in this example, is the limiting factor for the presence of the other species. It is very difficult, if not impossible, to design control treatments for these kind of interactions in the field. Also, natural variation in the investigated stress factor, e.g. light availability, may be larger than the differences in the experimentally applied treatments.
Monitoring
Monitoring of parameters in the field as an alternative to laboratory and field experiments is especially useful if the response mechanisms of species and ecosystems to stress and their tolerance limits are sufficiently documented and that their effects may be predicted. In general, monitoring of field conditions may provide relevant information on baseline data of potential indicators and timescales and extend of natural variation of these indicators on which to build the design of laboratory and field experiments.
Depending on the species and the stress factor examined, experiments to determine the effects of the stress and tolerance limits may take several months up to years. Especially if the effects of repetitive stress or multiple stress factors are important, and if the recovery potential and duration from sub-lethal effects become critical, long-term experiments are unavoidable. Fig. 6 shows an example of species abundance as a function of equal periods of repetitive stress and non-stress intervals in which the species response to stress is faster than the species recovery response. This leads to a stepwise decline in abundance and an increase of the time needed to fully recover from the effect of stress. This will eventually lead to mortality.
The response of a species to stress is not always visible as a gradual decline in species abundance. Often, the abundance declines very slowly until the moment at which a sudden collapse occurs (Fig. 7). If a sudden collapse occurs it is too late to take protective measures. In these cases it is important to identify and monitor indicator parameters that predict the expected species response and that are measures for the nearness to collapse, and that, once changes in these indicators become visible, preventive measures are applicable.
2. Response Trajectories
Following the conceptual diagram of Figure 5, typical ecological and environmental parameters that could/should be investigated or monitored to determine species response trajectories are:
Ecophysiological
These kind of parameters focus on processes that take place at molecular, cellular or tissue level in the plant. Ecophysiological indicators play a role in metabolic biochemical and biophysical reactions and processes in the plant.
- Photosynthetic parameters (photosynthetic performance and photosynthetic efficiency, Maximum productivity (Pmax), Saturation irradiance (Isat: irradiance at which P = 0.5 Pmax), Compensation irradiance (Ic: irradiance at which production equals respiration), Pmax/Isat (as a measure of photosynthetic efficiency))
- Pigments concentrations (chlorophylls, carotenoids)
- Carbohydrate reserves
- Tissue nutrient concentrations (C, N, P)
In response to e.g. a reduction in light availability, as to balance energy production to energy consumption, a seagrass may lower its maximum productivity, saturation irradiance and compensation irradiance, increase its pigments concentrations, and/or mobilise its carbohydrate reserves. These changes may have consequences for tissue nutrient concentrations as well. Physiological indicators should be monitored on a daily (for molecular and cellular responses: photosynthesis parameters, pigments) to maximum weekly (tissue responses: carbohydrates, nutrients) basis.
Morphological
These kind of parameters focus on changes in the appearance of the seagrasses as an adaptation to environmental stress. Fig 8 shows the morphology of some characteristic seagrass designs. A single seagrass shoot typically sprouts from a vertical stem or (vertical) rhizome and consists of a number of leaves. Roots and rhizomes form the belowground plant parts.
- Above and below ground biomass
- Leaf length and leaf width
- Leaf growth rate
- Leaf turnover
- Number of leaves per shoot
- Number of shoots per unit of rhizome length
- Rhizome diameter
- Rhizome growth rate
In response to e.g. a reduction in light availability, as to balance energy production to energy consumption, a seagrass may lower its energy consuming below ground biomass and/or increase its above ground energy producing biomass. Related to this are changes in tissue specific production, growth and turnover rates. In response to e.g. increased sediment accumulation, the seagrass may increase vertical rhizome or leaf growth as to outgrow sediment accretion rates. The ability of a seagrass to do so depends on light availability and/or carbohydrate reserves. Changes in morphological appearance usually coincide with physiological changes. Morphological parameters should be measured on a weekly basis.
Community
These kind of parameters focus on changes in the presence and/or appearance of a seagrass species in a community / ecosystem:
- (Specific) seagrass abundance (seagrass cover, specific shoot density)
- Species composition
- Ecosystem health status (ability to recover from perturbations)
In response to stress, depending on the competitive capacities of one species compared to the other, the species composition of an ecosystem or community may change, leading to changes in the (relative) dominance of one species over the other. Community changes usually also involve morphological and physiological changes of a seagrass. Measuring the ecosystem health status in the field could be done by experimentally disturbing the system (e.g. remove plants from a part of the system) and assess how fast it is able to recover from the disturbance. Community parameters should be monitored on a weekly to monthly basis.
Environmental parameters
These kind of parameters focus on changes in the environmental that may have an effect on seagrass fitness and/or seagrass or community functioning:
- Light availability at photosynthetic surface (leaves) and the water surface
- Water temperature
- Turbidity in relation to light availability (suspended solid concentration, algae)
- Suspended sediment composition
- Sedimentation rate and composition
- Nutrient concentration and composition
- Salinity
- Redox
- Soil Sulphate, Sulphide, Fe2+, Fe3+ concentrations
- Soil organic content, microbial activity, organic load
In response to changes in environmental parameters, changes in ecophysiological, morphological and/or community parameters are to be expected. As a result of dredging, the suspended sediment concentration in the water column may increase, lowering light availability and/or increasing the sedimentation rate. It is also possible that nutrient and organic load change, which could lead to increased algae production lowering the light availability for seagrasses, or to changes in biogeochemistry related to toxic soil conditions.
In laboratory and field set-ups it is possible to experiment with light and nutrient availability, organic load and sedimentation rate as to assess species specific and/or community responses, or to assess the effect of different (relevant) combinations of multiple stress factors. For manipulative experiments with temperature and salinity, laboratory / mesocosm set-ups are advisable. Many environmental parameters can be monitored constantly using data loggers.
3. Iterative process
Step 1: Niche width and tolerance limits
The establishment of species response characteristics starts with exploring the species niche width and tolerance limits to an environmental parameter. Ideally these can be derived from ongoing field monitoring efforts, e.g. by surveying seagrasses along a depth gradient, from the intertidal until the maximum depth limit, and record as many as possible physiological, morphological, community and environmental parameters. Alternatively, this could be done through literature review or by designing laboratory / mesocosm studies. Seagrasses should be grown from seeds or transplanted from the field in controlled set-ups under a range of different environmental values until it is clear at which values the seagrasses die and at which values the seagrass abundance stabilises at new equilibriums. Prior to the experiment the plants should be allowed sufficient time to acclimate to the laboratory conditions. This can take up to several months. During the experiment the seagrass abundance should be recorded at regular time intervals as to identify the response times and the shapes of the response trajectories at different environmental parameter values.
Step 2: Indicator parameters for decline
The second step involves exploring suitable physiological and morphological indicator parameters for seagrass decline. This could also be done through literature review or, if no sufficient literature data are available, laboratory / mesocosm studies. In addition to recording abundance described for the experiment under step 1 other parameters (such as mentioned above) could be sampled. Step one and two should reveal the species niche width and a number of relevant physiological and/or morphological parameters that could serve as early warning indicators for seagrass loss.
Step 3: Determine recovery criteria
Once niche width and relevant physiological and/or morphological indicator parameters are known, the third step is to determine the recovery potential and recovery time of the seagrasses from environmental stress by submitting the seagrasses to a range of time trials for different stress levels, followed by a recovery period until the seagrass condition has returned to the starting (t0) values. Seagrass abundance and selected indicatorsshould be recorded at regular time intervals. This experiment could be done in laboratory / mesocosm set-ups or in the field. The advantage of field experiments is that the relevance of the selected indicators could simultaneously be tested for applicability under natural conditions and that also community level indicators may be identified. A disadvantage is possible and sometimes unavoidable environmental and ecological differences between stress period and recovery period, although this also gives valuable information about natural heterogeneity. Step 3) could be extended with repetitive stress and recovery treatments as to establish the impact of repetitive stress on recovery potential. Another possibility is to extend step 3) with multiple stress parameters, e.g. besides testing the effects of light reduction due to increased turbidity on seagrass abundance, the additional effects of increased sedimentation, which often co-occurs with increased turbidity, could also be tested. Step 3) gives threshold values for one or more relevant indicator parameters in relation to stress intensity and species response time and recovery potential.
Step 4: Monitoring
The final step involves monitoring of indicator parameters in relation to marine infrastructure projects.
Step 5: Adaptive Management Strategy
Based on the knowledge of species tolerance limits, species response and recovery time and recovery potential obtained from steps 1-4, an ecological relevant indicator value based adaptive management strategy could be designed for dredging and coastal engineering efforts that may cause seagrass systems to decline. The management strategy may set stress time – intensity thresholds for coastal operations parallel to a real-time indicator monitoring and feedback system for the potentially affected seagrass system. Each operation that is potentially harmful for sensitive coastal ecosystems should have a real-time monitoring and feedback system. Thresholds for stress time – intensity should be verified or investigated for each (new or other) coastal ecosystem.
Note:
The selection of an appropriate indicator parameter is critical and not necessarily the same for different seagrass species (e.g. temperate vs. tropical or climax vs. pioneer species) or for the same species in different environments (e.g. height in the (inter)tidal and coastal zone or in mono vs. multi specific meadows). If the occurrence of a species cannot be considered without its ecological context it may be more relevant to identify an indicator for community responses instead of species responses, in which case laboratory / mesocosm experiments are less suitable than field experiments. In any case, the usability of an indicator should be tested for the targeted species or community in the field in the potentially affected area first, before it can be used. The construction of an indicator database for different species, communities and environmental conditions in relation to different forms and intensities of stress factors may prove very instrumental and could be subject for further efforts within the Building with Nature concept.