Eutrophication is a common threat to the integrity of coral reefs as it can cause altered balance and integrity of the reef ecosystem. On the island Bonaire the former waste water treatment is limited which is a point of concern to the quality of the marine park. The reef of Bonaire faces nutrient input by various sources, of which enriched groundwater outflow from land is considered to be a substantial one. It is assumed that groundwater is enriched with nutrients e.g. due to leaking septic tanks.
In order to reduce the input of nutrients on the reef via enriched groundwater, a water treatment plant is being built on Bonaire. The treatment of sewage water is extended in 2012 with a sewage system covering the so called sensitive zone, the urbanised area from Hato to Punt Vierkant, including Kralendijk, the islands largest town. Based on the dimensions of the treatment plant and estimated connections to the plant, it is estimated that a total of 17.5 to 35 tonnes of nitrogen a year will be removed from the sensitive zone, and will not leach out to the sea. No estimates are known of the contribution of other sources to the total nitrogen load.
Limited information was available about concentrations of nutrients in the marine local environment and its eutrophic state. Therefore, Rijkswaterstaat asked IMARES to conduct a study on water quality aspects. The goal of this coastal monitoring study was to collect baseline water quality data to be able to study the impact of the water treatment plant in coming years. The following research questions are discussed based on the results:
- Are environmental safe threshold levels of water quality exceeded?
- Is temporal (over the years), or seasonal variation (November-May) of water quality observed?
- Does water quality vary among locations or regions in Bonaire?
- Based on experience and results, what are recommendations for future monitoring of water quality?
The study area was the west coast of Bonaire, and included 12 field locations. Water was sampled during early morning field trips at each location twice a year (May and November) starting November 2011 till May 2013. Indicators for water quality related to the nutrient status on the reef were selected and analyzed.
Based on their relevance to general water quality aspects and steering primary production, their relevance to the outflow of enriched (polluted) groundwater (and thus possible impact of the treatment plant in future) the following indicators were included:
- Inorganic nutrients
- NO2, NO3, NH4, PO4
- DIN (calculated based on NO2+ NO3+ NH4)
- Organic nutrients
- Total nitrogen, ureum and total phosphorus
- General water parameters
- Fecal bacteria
Concentrations were assessed against environmental threshold values from peer reviewed literature or (inter)national standards. If not available, outlying concentrations were highlighted taking the 80th percentile as a representative level.
Results and discussion
Water quality indicators measured at the west coast of Bonaire show signals of eutrophic conditions. Spatial and temporal variation in water quality is however observed. At some locations and certain moments environmental safe levels of nutrients are exceeded (see overview of data in Figure 1- Figure 4). Especially at locations in the south and in the sensitive zone concentrations of nitrogen and phosphorus exceed the threshold levels. Southern locations are probably affected by the salt pans, and locations in the sensitive zone by outflow of sewage water.
Furthermore, an increase of phosphorus and chlorophyll-a is observed in the last 2 years, whereas nitrogen (DIN) decreases slightly over the years. However, despite the decrease of nitrogen, its threshold levels are exceeded at Red Slave, Tori’s reef, Angel City, 18th Palm, Cliff. Phosphorus and chlorophyll-a do not yet exceed environmental threshold levels, but if the increase continues, this might be relevant in near future.
The risk of higher nutrient levels is that algal growth can outcompete corals, and can change the structure of the ecosystem. Furthermore, increased levels of nutrients affect the coral reefs integrity due to decreased stability of the skeleton.
The increase of bioavailable phosphate alters the nutrient ratio (DIN:SRP ratio) and species composition can evolve from this change in relative nutrient availability. Relating these data with observations in benthic composition and chlorophyll-a trends is advised to support this hypothesis.
Fecal bacteria numbers exceed several standards for human health safety. High fecal bacteria numbers are more frequently found in the south and in the sensitive area, and are likely to be related to rainfall events. Bacteria are found in surface samples as well; indicating surface run off as a possible source.
Actual rainfall, especially just before or during sampling is an important steering factor in the concentrations measured. Rainfall is very scattered during the rainy season, and we believe so is the outflow of nutrients to the reef.
In short it is recommended to continue the monitoring of water quality over several years at the same frequency and locations. Next to the regular program, make sure that interval sampling during heavy rains are included as these moments indicate point source discharges which can be missed when rainy season is shifted. No locations should be discarded from the program. In order to prepare the monitoring program for future measures taken outside the current zone (Hato- Punt Vierkant) additional locations just north and south of the sensitive zone are advised to be included. The set of indicators can remain the same, with some slight adaptations such as the addition of coprostanol (measure of faecal discharge) and discard of ureum.
As nutrient levels are in a constant flux, data should be considered in an ecosystem context. Benthic surveys focusing on macro algae, turf algae and cyanobacteria, were not included in this study, but add largely to a whole ecosystem assessment on eutrophication issues.
Monitoring of water quality in the coastal zone alone will not provide satisfactory indication of the impact of the treatment plant in reducing emissions to the marine environment. To monitor the impact of the treatment plant, several factors should be considered. These are related to the treatment plant itself, groundwater quality, coastal water quality, benthic coverage and benthic quality. Actual reduction of emissions to the marine environment can be retrieved from monitoring and reporting of the efficiency of the treatment plant. Monitoring of groundwater wells provides knowledge on the groundwater quality that outflows to the reef. Water quality monitoring in the coastal zone gives knowledge on conditions contributing to environmental health. It is advised to synchronize the monitoring programs, and to analyze the datasets in a coherent way.
In the end, eutrophication is not the only pressure potentially affecting a reef. Besides the focus on the research related to the treatment plant it is advised to consider additional research on a “whole ecosystem basis” in which the contribution of other pressures as well, such as run off via canals and overflows of salinas with nutrients and sediments (in rainy season), fisheries impact and the impact of climate change/acidification on the reef are included.