- Singapore mesocosm experiment
- Field experiments in Singapore and Indonesia
- Conclusion: Best practice to set safe site-specific dredging criteria
1. Singapore mesocosm experiment
In Singapore the effects of 65 days light reduction and 30 days of elevated temperature was investigated on the tropical seagrass species Halodule uninervis in an outdoor mesocosm experiment at St Johns Island (Nilmawati, 2012). Physiological and morphological parameters were measured to identify possible (early warning) indicators for seagrass decline in response to light and temperature stress. Three different levels of light (Control: 100% Surface Irradiance (SI), moderate (MS): 60%SI, and high shading (HS) 40% SI) were applied using shading screens from day 0 to day 65 and two different temperatures (29oC (day 0 to 65) and 34oC (day 35 to 65)) were applied using aquarium heaters to examine the impact of light and temperature change on physiological and morphological parameters. The chosen light and temperature levels and candidates for indicators were based on literature and monitoring data. Actual light availability followed the natural variation in light levels. Temperature and light levels were recorded every 30 minutes. It is expected that plants will respond to shading with adaptations that allow balancing photosynthetic energy production to respiratory energy consumption. Increased temperature is expected to cause increased respiratory energy consumption, putting additional stress on balancing the plants’ energy budget.
The light levels used in the mesocosm experiment were compared with literature values for minimum light requirement for Halodule uninervis in the field. Collier et al. (2012) used three ways to express constants for minimum light requirements for Halodule uninervis, taking adaptations to different location specific light regimes into account. These are i) the mean daily irradiance (Id) reaching a minimum value, ii) the percentages of days that a certain daily irradiance value is reached, and iii) the minimum value of hours per days that saturating light irradiance (Isat) is reached (Hsat). Table 1 shows these minimum light requirement constants and the concurring estimates of light availability in the three mesocosm light treatments, assuming that plants have (physiologically) adapted to their respective light treatments. The comparison suggests that HS light availability is at minimum light requirement for one, and below minimum light requirement for two of the tree indicators. It is expected that (prolonged) HS shading will eventually lead to die-off of this seagrass.
Table 1. The mean daily irradiance (Id), 16-18% daily irradiance, and hours saturating irradiance (Hsat) as values for minimum light requirements for Halodule uninervis from Collier et.al (2012) and the corresponding estimates from a mesocosm study for control (100% Surface Irradiance – SI), MS (60% SI) and HS (40% SI) treatments, respectively. Standard deviation in brackets.
|ld (mol m-2 d-1)||5||10 (4)||6 (3)||5 (2)|
|16-18% daily irradiance (mol m2 d-1)||3||5 (1)||3 (0.3)||2 (0.4)|
A significant reduction in leaf width, leaf length, shoot density, number of leaves per shoots, rhizome diameter and rhizome length relative to the starting material was found after 65 days in all light treatments, including the control. Leaf growth and above and below ground biomass also changed in all treatments during the course of the experiment. The morphological parameter results, however, did not differentiate the effects of the light and temperature treatments on seagrass development in this experiment.
Chlorophyll a and b increased in response to shading. Chlorophyll fluorescence showed a pattern that is opposite to the natural irradiance level, but was consistently higher at HS compared to MS and control. Higher chlorophyll fluorescence and higher chlorophyll a and b contents at high shading may indicate a physiological compensation strategy to low light availability as to optimise photosynthetic efficiency, pointing to a survival strategy balancing energy production to energy consumption.
Temperature increase (35 to 65 days) showed a decline in photosynthetic efficiency, probably as a result of a higher respiration rate at 34oC compared to 29oC. The level of stress (light reduction and temperature increase) thus induced a physiological change, the first stage in the response sequence.
Lessons Learned Singapore Mesocosm experiments:
- The results indicate that under the environmental conditions of the mesocosm treatments, Halodule uninervis survives for at least 65 days.
- The measured morphological changes were probably the result of transplanting the plants from the field to the mesocosms and handling of the plants during the experiment, rather than the result of light reduction or temperature increase. The morphological parameters in this experiment could not be used as indicators of change in response to the light and temperature treatments.
- It is clear that the experimental set-up itself provoked significant morphological responses, which illustrates the potential drawbacks of mesocosm / laboratory experiments over field experiments.
- The physiological parameters chlorophyll a and b contents and chlorophyll fluorescence did show a difference in response between seagrasses growing under different mesocosm conditions.
- Chlorophyll a and b contents can be used as indicator if the (natural) plants’ chlorophyll content is still well below the maximum (or optimum) chlorophyll content. Leaves with a ‘low’ natural chlorophyll content, which was the case in this experiment, are able to increase light capturing efficiency by increasing the chlorophyll content, while leaves with a ‘high’ (maximum) natural chlorophyll content will not profit from increasing the chlorophyll content.
- Large fluctuations in chlorophyll fluorescence following the variations in natural light availability makes chlorophyll fluorescence difficult to use as an indicator in the field. Chlorophyll fluorescence is highly variable (i.e. response time is too short) and is therefore not a practical indicator in the field. The assessment of location and species specific minimum light requirement in combination with light measurements in the field may prove to be a useful indicator predicting the development of seagrass abundance.
- It is advised to i) allow seagrasses to fully adapt to the mesocosm environment until a situation is reached where the number seagrass shoots, the number of leaves and the leaf lengths and widths is stable before commencing experiments, ii) to use measuring techniques that do not negatively affect the fitness of the plants and iii) to run the experiment until a new steady state is reached in all treatments (including complete die-off).
2. Field experiments in Singapore and Indonesia
Within the context of the Singapore Delft Water Alliance (SDWA), several studies on long-term shading treatments were made, both on fast-growing pioneer species as well as slow-growing climax species. In addition to applying long-term shading, also the effect of additional disturbances were tested.
Below a brief summary is given of the main conclusions from the research on the sensitivity of seagrass to turbidity; detailed results will be published separately. The preliminary conclusions from the research so far are:
- Long-term monitoring showed that seagrass meadows in turbid environments like Singapore are surviving close to or at the limits of their tolerance to low light. Additional shading from enhanced turbidity will further decreases seagrass health (Yaakub et al. in prep 1).
- Experimental fieldwork showed that it is possible to test the effect of additional shading on seagrass meadows growing in turbid environments, by applying a novel multi layer shading approach (Yaakub et al. in prep 2).
- Experimental fieldwork showed that increasing turbidity with a similar level may have markedly different effects, depending on the initial health status of the seagrass meadow (Yaakub et al. in prep 2).
- Experimental fieldwork showed that additional turbidity-related shading makes seagrasses increasingly sensitive to other kinds of disturbances, and may cause mortality, where this would not have happened in the absence of this extra turbidity-related shading (Yaakub et al. in prep 3).
- Experimental fieldwork shows that recovery following a disturbance is generally faster in pioneer than climax vegetations, irrespective of shading treatment, and that shading does not seem to cause shifts in species composition (Yaakub et al. in prep 4).
- Long-term shading experiments are extremely hard to carry out in the field due to disruptions from monsoons and other natural phenomena, but such experiments are essential in determining the effect of shading on slow-growing species. As long as the shading is not lethal, plants appear able to recover if original light conditions are restored (Yaakub et al. in prep 5), unless other disturbances interact (Yaakub et al. in prep 3 & 4).
- The shape of a disturbance has consequences for the rate of seagrass recovery (Yaakub et al. in prep 6).
Overall, this work emphasizes that response curves to shading should NOT be looked upon as a fixed relationship; the response is strongly related to the existing condition of the meadow and the probability of the co-occurrence of other disturbances. Both aspects (poor initial health & presence of additional disturbances) make seagrass meadows extremely vulnerable to any additional turbidity-related shading.
Lessons learned – field experiments in Singapore and Indonesia:
The studies clearly demonstrate / have taught us:
- How to set-up long-term shading experiments in highly turbid areas by using a novel multi layer shading approach, and the great value of the results of such experiments
- The importance of accounting for the pre-conditioning of a seagrass meadow to assess the impact of further reduction of light conditions
- The importance of studying the effect of shading on the resilience to additional disturbances as can frequently occur in natural seagrass ecosystems, in order to assess the sensitivity of the seagrass ecosystem to further reduction of light conditions
3. Lessons learned – Best practice to set site-specific safe dredging criteria
The mesocosm studies in Singapore and the field studies showed that the response of a seagrass species and of a seagrass meadow to (additional) light reduction as caused by e.g., dredging, is strongly related to i) the level to which seagrasses are adapted to prevailing environmental conditions ii) the existing condition of the meadow prior to dredging and iii) the probability of the co-occurrence of other disturbances. The latter implies that it is difficult to work with general literature based species response curves, as these general data must always be based on the worst case scenario to be safe.
To set realistic turbidity criteria for a dredging operation, the following best practice is recommended:
- Place cheap light loggers near the seagrass meadow to obtain insight in the temporal variability in light availability and/or turbidity events. Especially in places where dredging is regularly re-occurring, it would be highly beneficial to collect long-time light measurements near the seagrass leaves, to be able to select the optimal dredging periods
- To account for the local seagrass health of the meadow that will be impacted by the dredging plume, one needs to set-up on the dredging site a small-scale shading experiment.
- In case the turbidity level that will be generated by the dredging is known, but the impact on the seagrass over time is unknown, the following experiment is advised. The shading should mimic light reduction of 1.25 x foreseen dredging turbidity. To achieve this one could very well use the novel developed multi layer shading approach. Exaggerating the light reduction by 25% provides a safety margin in predicting the dredging impact on the seagrass. Seagrass health should be monitored over time by measuring cover, specific shoot density and species composition. 5 or more replicate shading frames are recommended.
Alternatively and in addition, one could determine the minimum light requirements of (the most relevant) seagrass species in the ecosystem and compare these with the light availability under the planned dredging conditions. If the light availability is close to or lower than the plants’ minimum light requirements, a sequence of stress and recovery trails is advised as to determine the maximum stress exposure time that will still allow for seagrass recovery (e.g. with field experiments as described above). The planning of the dredging operation should stop before the maximum exposure time is reached, taking a realistic safety margin into account.
- In case the turbidity level that is allowed during dredging needs to be established, but the dredging period is known, the following experiment is advised. A range of shading levels should be created by reducing light level at the seagrass bed by 25%, 50% and 75%. To achieve this one could very well use the novel developed multi layer shading approach. The shading period should be 1.25 x the dredging period, to include a 25% safety margin in predicting the dredging impact. Seagrass health should be monitored over time by measuring cover, specific shoot density and species composition. 5 or more replicate shading frames are recommended per light level.
Alternatively, one could determine the minimum light requirements of (the most relevant) seagrass species in the ecosystem and expose the segrasses for the intended length of the dredging period to light levels below the minimum light requirements and assess the seagrasses’ recovery potential and recovery time.
- In case both the turbidity level that is allowed during dredging, and the dredging period needs to be established the same experiment is recommended as described before (i.e., 25%, 50% and 75% light reduction at the seagrass leaves) and to monitor seagrass health for a period that is at least 1.25 x the period that the dredging could take place
Alternatively a sequence of stress and recovery trails below minimum light requirements is advised as described above. This assumes that if light availability under dredging conditions is above the minimum light requirements (taking a safety margin into account), seagrass will recover given sufficient recovery time is available.