A baseline water quality assessment of the coastal reefs of Bonaire, Southern Caribbean

Marine Pollution Bulletin 86 (2014) 523–529

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Baseline

Edited by Bruce J. Richardson The objective of BASELINE is to publish short communications on different aspects of pollution of the marine environment. Only those papers which clearly identify the quality of the data will be considered for publication. Contributors to Baseline should refer to ‘Baseline—The New Format and Content’ (Mar. Pollut. Bull. 60, 1–2).

A baseline water quality assessment of the coastal reefs of Bonaire, Southern Caribbean Diana M.E. Slijkerman a,⇑, Ramón de León b, Pepijn de Vries a a b

Wageningen UR – IMARES, Netherlands STINAPA Bonaire, Netherlands

a r t i c l e

i n f o

Article history: Available online 17 July 2014 Keywords: Tropical marine ecosystems Caribbean Nutrients Eutrophication Treatment plant

a b s t r a c t Bonaire is considered to harbor some of the best remaining coral reefs of the Caribbean, but faces multiple pressures including eutrophication. We measured multiple water quality indicators twice annually, from November 2011 to May 2013, at 11 locations at the west coast of Bonaire. This study resulted in 834 data points. DIN concentrations ranged from below quantification to 2.69 lmol/l, phosphate from below quantification to 0.16 lmol/l, and chlorophyll-a from 0.02 to 0.42 lg/l. Several indicators showed signs of eutrophication, with spatial and temporal effects. At southern and urban locations threshold levels of nitrogen were exceeded. This can be a result of brine leaching into sea from salt works and outflow of sewage water. Chlorophyll-a showed an increase in time, and phosphorus seemed to show a similar trend. These eutrophication indicators are likely to exceed threshold levels in near future if the observed trend continues. This is a cause for concern and action. Ó 2014 Elsevier Ltd. All rights reserved.

The coral reefs of Bonaire- southern Caribbean- are considered to be among the healthiest and most resilient in the Caribbean (IUCN, 2011; Perry et al., 2013). IUCN (2011) recently assessed the resilience of the reefs of Bonaire and highlighted some of the main threats to their ecological resilience and concluded that nutrient enrichment may be a key factor in coral decline. Bak et al. (2005) and Stokes et al. (2010) demonstrated a long-term decline of live coral cover on the reefs of Bonaire and an increase of macro-algae. In the Caribbean many factors impacted coral health and cover, such as the mass mortality of Diadema antillarum (e.g. Bak et al., 1984; Debrot and Nagelkerken, 2006), coral bleaching events (e.g. Wilkinson and Souter, 2008), diseases (Aronson and Precht, ⇑ Corresponding author. E-mail address: [email protected] (D.M.E. Slijkerman). http://dx.doi.org/10.1016/j.marpolbul.2014.06.054 0025-326X/Ó 2014 Elsevier Ltd. All rights reserved.

2001), and storm damage (e.g. Scheffers and Scheffers, 2006). Furthermore, land based pressures add to coral stress via nutrient discharge and sedimentation. Because coral reefs thrive in oligotrophic conditions, eutrophication is a serious point of concern. In general, enhanced nutrient levels may lead to eutrophication effects resulting in an altered balance of the reef system via various pathways. When nutrient concentrations exceed a certain threshold value, algae might outcompete corals (Lapointe, 1997). Additionally, nutrients contribute to the deleterious impacts of coral diseases and bio-eroding organisms (Carreiro-Silva et al., 2009; Mueller et al., 2014), and eventually affect the integrity of coral growth and calcification (Fabricius, 2005). In turn these aspects reduce reef resilience to climate change. Related to climate change, increased storms and hurricanes can accelerate reef degradation. The reefs of Bonaire are recognized as being among the best remaining coral reefs of the Caribbean with high biodiversity

D.M.E. Slijkerman et al. / Marine Pollution Bulletin 86 (2014) 523–529

(Hawkins et al., 1999; Hawkins and Roberts, 2004; Stokes et al., 2010). The reefs have been relatively well-studied for its reefs and fish, but almost no peer reviewed data are available on reefwater nutrient concentrations. Since the coral reef is a crucial contributor to the economic value of Bonaire through fisheries and dive tourism (Cado van der Lely et al., 2013), data on eutrophic conditions are essential for both research context and management of Bonaire’s marine ecosystem values. IUCN (2011) and STINAPA (Stichting Nationale Parken, the National Parks Foundation of Bonaire) advised strongly to invest in controlling pressures that are present in Bonaire. An assumed dominant pressure is nutrient enriched groundwater outflow from leaking septic tanks. A total of 118.275 m3/year of enriched groundwater has been estimated to flow into the reef ecosystem, from hotels alone, while residential properties and businesses certainly also contribute significantly (Van Slobbe, 2008). However, no data on water quality and eutrophication in the coastal zone is reported in peer reviewed literature. The purpose of this study was to collect and present data on water quality in coastal reef waters of Bonaire. Water quality indicators were assessed against generic safe threshold levels for eutrophication. Temporal and spatial patterns are analysed in order to identify the dynamics in water quality. Bonaire is a small tropical island of 38 km long and 5–8 km wide. Bonaire’s soil system is karstic, and channels lead to outflow of groundwater to the reef ecosystem. Bonaire is characterised by a dry and rainy season. Rain events are concentrated from November–January. In total 11 sites were selected along the west coast of Bonaire, including two off the small island of ‘‘Klein Bonaire’’ situated 1 km west of the capital settlement of Kralendijk (Table 1 and Fig. 1). The selection of locations included the urban area of Kralendijk where sewage water is considered to flow directly out onto the reef. Improvement of the water quality is expected in this area due to the installation of the treatment plant. These locations are situated between ‘‘Hato and Punt Vierkant’’ (see Fig. 1). Additional locations in the south, north and Klein Bonaire were selected to explore spatial variation. From November 2011 till May 2013 four sampling sessions were conducted, covering both dry and rainy season to observe any temporal variation. A selection of indicators was used to provide a first impression on eutrophication (Table 2).  + 3 These were the inorganic nutrients NO 3 , NO2 , NH4, PO4 (SRP-sol  uble reactive phosphorus), DIN (sum of NO3 , NO2 and NH+4), and organic nutrients (total nitrogen, total phosphorus). Chlorophylla was selected as an early warning indicator on the impact of nutrients to environmental change. Water was sampled at the beginning of the reef slope near the bottom by using SCUBA. At each sampling point, three poly-ethylene bottles of 500 ml (HCl and Milli-Q rinsed according to protocol) were filled for nutrient analysis. Three 1 l dark bottles were filled for chlorophyll-a analysis. General water quality parameters were

Table 1 Overview of locations and there position.

Playa Funchi Karpata Cliff Front Porch Playa Lechi 18th Palm Angel City Tori’s Reef Red Slave South Bay Ebo’s special

Latitude

Longitude

Depth reef slope (m)

12°16’56.5400 N 12°130 9.1400 N 12°100 27.7600 N 12°100 1.1300 N 12°90 27.2000 N 12°80 18.8500 N 12°60 11.2000 N 12°40 13.9800 N 12°10 34.6700 N 12°80 58.5800 N 12°90 56.3600 N

68°240 50.2800 W 68°210 6.4200 W 68°170 24.6600 W 68°170 13.8100 W 68°160 48.0400 W 68°160 34.8200 W 68°170 13.6400 W 68°160 50.1400 W 68°150 3.6700 W 68°190 13.2700 W 68°190 9.5700 W

10 5–6 6–7 7 8–9 7 5–6 8 15–17 8–9 8

Trade wind 12°18′

Playa Funchi 12°15′

Karpata

Latitude (N)

524

12°12′

Hato Cliff Front Porch Playa Lechi

Ebo’s Special 12°09′

South Bay

18th Palm Punt Vierkant

Angel City

12°06′

Tori’s Reef 12°03′

Red Slave

Caribbean Sea 68°24′

68°18′

68°12′

Longitude (W) Fig. 1. Geographical overview of Bonaire with locations sampled. Hato and Punt Vierkant represent the outer limits of the zone in which sewage water will be transported to the treatment plant.

measured in situ in one dark bottle, using a YSI-6600 multi parameter probe (dissolved oxygen, salinity, temperature, pH). Samples were stored in cool boxes with ice packs. Processing of collected samples in the laboratory was conducted within three hours after sampling. All vials and filters used in processing of nutrient samples were three times HCl rinsed. Of each of the three bottles for nutrients, one sample for inorganic nutrients was processed by filtering the sampled water over a 0.2 lm disposable filter. The vial was prerinsed three times with the filtered sample before taking the actual sample. Organic nutrient samples were not filtered, and processed with the same rinsing procedure. Nutrients samples (volume 20 ml) were deep-frozen at 20 °C and transported to the Netherlands for analysis. Samples were analysed within 1 month using aQuAAtro continuous flow analyser, according to methods described by Searle (1984) for nitrogen (NH+4, NOx), and JGOFS (1994) for ortho-phosphate. Total nitrogen and total phosphorus samples were destructed and analysed according to principles by Valderrama (1981). Quantification limits are presented in Table 3. 1 litre chlorophyll-a samples were filtered onto a fibre filter and stored deeply frozen at 20 °C. Samples were analysed using acetone extraction within 14 days (method according to Holm-Hansen et al., 1965). Our data set amounts a total of 834 individual samples of water quality parameters (specifications provided in Table 2). Data of all parameters were fourth root transformed and a two-way ANOVA (ANalysis Of VAriance) was performed. We compared indicators between seasons, locations and time. Season refers to the differences between wet and dry season, whereas time refers to the observed difference between 2011 and 2013 (taken as November 2011 and May 2012 vs. November 2012 and May 2013). As this is only a two year study, data on season and time are limited and its significance should be interpreted as preliminary indications. In addition, locations were also grouped to regions (southern (Red Slave, Tori’s Reef, Angel City), urban (18th Palm, Playa Lechi, Front Porch, Cliff), northern (Karpata, Playa Funchi), Klein Bonaire (Ebo’s Special, South Bay)) to be able to evaluate differences in water quality at a larger spatial scale. ANOVA analyses were followed by a post hoc Tukey’s test to assess pairwise group

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Table 2 Details of sampling period and analysis. * DIN is calculated based on NO2 + NO3 + NH4. Total number of gained samples summed to 834. Sampling

Year

Month

Analyses

Total samples

NO 2,

NO 3,

PO3 4 ,

1

2011

November 11, 13–17

NH 4,

2

2012

May 24–27

3

2012

4

2013

November 19–22 May 27–30

  3 NH 4 , NO2 , NO3 , DIN, PO4 , total nitrogen, total phosphorus, chlorophyll a   3 NH 4 , NO2 , NO3 , DIN, PO4 , total nitrogen, total phosphorus, chlorophyll a   3 NH 4 , NO2 , NO3 , DIN, PO4 , total nitrogen, total phosphorus, chlorophyll a

DIN,

chlorophyll a

Table 3 Quantification limits (lmol/l) for each of the measures parameter. DIN = sum of NH-4 and NOx. Parameter

Limit (lmol/l)

NO 2  NOx (NO 2 + NO3 ) NH 4 PO3 4 Total P Total N

0.015 0.055 0.08 0.035 0.1 1.24

differences. All statistical analyses have been implemented and executed in R version 3.1.0 (The R Foundation for Statistical Computing, Vienna). The data is displayed using box plots in which the bold line indicates the median value for that specific factor, while the boxes indicate the first and the last quartile of the data Whiskers indicate the minimum and maximum values, excluding outliers. Outliers are shown with markers (°). In the box plots, data are considered to be outliers if they deviate more than 1.5 times the interquartile range from the first or third quartile. Concentrations measured below the limit of quantification were cut off at that limit. The reported concentrations could thus be an over-estimation and will lead to higher average values and smaller standard deviation. The number of values below the limit of quantification are included in Table 5. Per sampling event, the mean value of each indicator (per location) was calculated and assessed against the best available threshold values. These values reflect the risk for eutrophic algal growth potentially outcompeting coral growth, as defined by Bell (1992) and Bell et al. (2013) (Table 4). For total nitrogen and total phosphorus, environmental threshold values are not available. To construct a local threshold value the approach by the Queensland Australia (Queensland Government, 2009) was followed. The 80th percentile of all retrieved data per indicator- after cutting off at the quantification limit- in this monitoring period was taken. In Table 4 the applied threshold values are presented. Fig. 2 shows all indicators as presented in boxplots. Table 5 provides an overview of all average concentrations including standard deviation per location per sampling moment. The number of measurements below the limit of quantification is presented in the table. In Fig. 3 a graphical summary of data is presented, including the relative

10 locations * triplicate samples per indicator. 1 set of 6 values was discarded due to obvious pollution during processing (1 bottle nutrients, 1 bottle for Chlorophyll-a) = 174 values 11 locations * triplicate samples per indicator (as 2011 set) + single values for tN and tP = 220 values. No sample was lost. Same as May 2012 Same as 2012

risk of the exceeded threshold levels per indicators in sequential grey scales. In our study, DIN ranged from 0.01 lmol/l to 2.69 lmol/l. DIN concentrations varied significantly across locations, with two locations (18th Palm and Angel City) standing out for significant higher concentrations than Playa Funchi (p = 0.01). When comparing the different regions in the coastal zone, northern locations had significantly lower DIN concentration than locations in the urban area and south (p < 0.05). DIN concentration also showed seasonal differences, with average November concentrations (0.66 ± 0.54 lmol/l) being slightly lower (p < 0.05) than concentrations in May (0.73 ± 0.45 lmol/l). Data show significant temporal fluctuations, but no consistent pattern. In general, DIN concentrations showed a slight decreasing trend over time (p < 0.001) when 2011–2012 is compared with 2012–2013 data. More data points in time are needed to confirm a substantial trend over time and differences between seasons. The applied threshold value for nitrogen of 1 lmol/l was occasionally exceeded at locations in the south and urban area (Red Slave, Angel City, Tori’s Reef, 18th Palm, Playa Lechi, Cliff, Ebo’s Special). Angel City is a location at which DIN appeared to consistently exceed the threshold value. Ammonium concentration ranged from below the limit of quantification to 2.31 lmol/l. The concentration did not vary between the seasons. November had an average concentration of 0.48 ± 0.52 lmol/l and May an average of 0.35 ± 0.33 lmol/l. Ammonium concentration was significantly lower during the 2012–2013 samplings compared to the 2011–2012 samplings (p < 0.001). Northern locations had lower average concentrations than the southern and urban locations (p < 0.05). Concentrations of ammonium exceeded the threshold value for nitrogen occasionally at Red Slave, Angel City, Tori’s Reef, Playa Lechi, and Cliff. Except for Playa Lechi, all other observations refer to the sampling of November 2011. Nitrate concentrations ranged from below the limit of quantification to 1.31 lmol/l. Nitrate varied significantly among locations (p < 0.001), time (p < 0.05) and season (p < 0.001). In November the average nitrate concentration was 0.20 ± 0.14 lmol/l, and in May 0.38 ± 0.22 lmol/l. The latter average was steered by high values in May 2013 compared to the previous sampling moments. Playa Funchi in the north of Bonaire showed significantly lowest nitrate concentrations. No significant differences between coastal

Table 4 Water quality standards for applied indicators. Indicator

Applied environmental threshold

Reference

Inorganic nutrients Organic nutrients

DIN: 1 lmol/l, PO3 4 ,: 0.1 lmol/l

Bell (1992)

Total nitrogen: 9.39 lmol/l Total phosphorus: 0.21 lmol/l 0.2 lg/l

Based on 80th percentile of this dataset. According to guidelines provided by Queensland Government (2009)

Chlorophyll-a

Bell et al. (2013)

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D.M.E. Slijkerman et al. / Marine Pollution Bulletin 86 (2014) 523–529

November

May

1.5

3.0

NH4 (µmol N−NH4/l)

DIN (µmol/l)

2.5 2.0 1.5 1.0

NO3 (µmol N−NO3/l)

2.5 2.0 1.5 1.0

0.5

0.5

0.0

0.0

1.0

0.5

0.0

Chl−a (µg/l)

0.4

2013 May

2012 November

0.5

0.2

0.0 2012 May

0.00 2011 November

0

0.05

2013 May

5

0.4

2012 November

10

0.10

2012 May

15

0.6

0.15

2011 November

20

Total P (µmol P−PO4/l)

Dissolved P (µmol P−PO4/l)

Total N (µmol N−NO3/l)

25

0.3

0.2

0.1

2013 May

2012 November

2012 May

2011 November

0.0

Fig. 2. Nitrate concentration, Ammonium, DIN and PO4 (lmol/l) in time, reported for months November and May, based on all locations (n = 11, except for 2011 n = 10), median values presented, whiskers indicate minimum and maximum values, excluding outliers (marked by circles). Grey dotted line represents the limit of quantification. The grey striped line represents the threshold value (nitrogen, being 1 lmol/l, PO4 = 0.1 lmol/l, chlorophyll-a = 0.2 lg/l).

regions were observed. Nitrate showed a slight increase over the years, but whether this trend is structural and of any ecological relevance should be confirmed with additional data in time. Total nitrogen ranged from below limit of quantification to 21.06 lmol/l. No differences in time nor in season was observed. Among locations slight differences were observed. South Bay had significantly lower concentration of total nitrogen in May 2012 compared to all other locations (p < 0.05). This is however only observed at that moment. Locations with highest values (>1 or 1.5 the 80 percentile) included Playa Funchi, Karpata, Playa Lechi, 18th Palm and Angel City. Phosphate concentrations ranged from below limit of detection to 0.16 lmol/l. The environmental threshold value of 0.1 lmol/l was only rarely exceeded (18th Palm). Spatial and temporal significance of variation in phosphate concentrations could not be tested due to the high number of concentrations under the limit of quantification. Most unquantifiable data were found at the start of monitoring in November 2011. The concentration of 22 out of 30

samples could not properly be quantified. In time, more concentrations could be quantified (table 5), and in May 2013 only 3 out of 33 samples were unquantifiable. Because the lack of proper statistics, we speculate that phosphate concentrations increased during our study period (Fig. 2). If this increase in concentration continues the threshold value for phosphate (0.1 lmol/l) will certainly be exceeded in near future. Locations Front Porch and 18th Palm are suspect locations as they showed highest concentrations during this study. Total phosphorus ranged from below limit of quantification to 0.61 lmol/l. Locations showing occasionally highest total phosphorus concentrations (>1 or 1.5 the 80 percentile) include Red Slave, Angel City, Playa Lechi, Front Porch, Cliff and Karpata (see Fig. 3). Chlorophyll-a concentrations ranged from 0.02 to 0.42 lg/l and varied among locations (p < 0.001). Playa Lechi showed higher concentrations than Angel City and Tori’s reef. The threshold was occasionally exceeded (Playa Lechi, Red Slave). No significant variation

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Table 5 Overview of concentration per indicator for each of the locations at all four sampling moments. Presented as mean values ± standard deviation. NA = not available. Between parentheses are the number of replicates below the limit of quantification. NH 4 (lmol N–NH4/l)

NO 3 (lmol N–NO3/l)

DIN (lmol N/l)

PO3 4 (lmol P–PO4/l)

0.41 ± 0.37 0.26 ± 0.26 (1) 1.22 ± 0.29

0.01 ± 0.01 0.32 ± 0.07 0.09 ± 0.04

0.42 ± 0.38 0.56 ± 0.30 1.31 ± 0.30

0.035 ± 0.000 (3) 0.037 ± 0.003 (2) 0.037 ± 0.003 (2)

0.137 ± 0.006 0.150 ± 0.023 0.155 ± 0.015

0.58 ± 0.57 0.55 ± 0.01 0.14 ± 0.10 (2) 0.44 ± 0.33 (1) 1.46 ± 0.88 1.01 ± 0.66 1.14 ± 0.50

0.40 ± 0.02 0.04 ± 0.04 0.25 ± 0.05 0.29 ± 0.05 0.21 ± 0.02 0.15 ± 0.03 0.09 ± 0.01

0.98 ± 0.57 0.59 ± 0.06 0.36 ± 0.10 0.72 ± 0.40 1.66 ± 0.89 1.16 ± 0.69 1.23 ± 0.49

0.035 ± 0.000 0.035 ± 0.000 0.035 ± 0.000 0.040 ± 0.000 0.035 ± 0.000 0.035 ± 0.000 0.042 ± 0.011

(3) (2) (3)

0.103 ± 0.023 0.164 ± 0.025 0.122 ± 0.033 0.127 ± 0.010 0.073 ± 0.014 0.102 ± 0.018 0.097 ± 0.011

May 2012 Playa Funchi Karpata Cliff Front Porch Ebo’s Special Playa Lechi South Bay 18th Palm Angel City Tori’s Reef Red Slave

0.28 ± 0.10 0.38 ± 0.27 0.28 ± 0.09 0.31 ± 0.06 0.17 ± 0.03 1.10 ± 0.85 0.20 ± 0.08 0.57 ± 0.10 0.65 ± 0.59 0.25 ± 0.08 0.26 ± 0.07

0.18 ± 0.03 0.37 ± 0.04 0.24 ± 0.01 0.26 ± 0.02 0.21 ± 0.02 0.17 ± 0.02 0.16 ± 0.01 0.22 ± 0.03 0.58 ± 0.35 0.32 ± 0.03 0.55 ± 0.18

0.46 ± 0.12 0.76 ± 0.24 0.52 ± 0.10 0.57 ± 0.08 0.38 ± 0.03 1.28 ± 0.86 0.36 ± 0.07 0.79 ± 0.09 1.24 ± 0.87 0.58 ± 0.10 0.82 ± 0.25

0.035 ± 0.000 0.035 ± 0.000 0.039 ± 0.004 0.047 ± 0.007 0.035 ± 0.000 0.036 ± 0.001 0.039 ± 0.007 0.041 ± 0.010 0.041 ± 0.010 0.063 ± 0.048 0.059 ± 0.011

(3) (2) (1)

2012 November Playa Funchi Karpata Cliff Front Porch Ebo’s Special Playa Lechi South Bay 18th Palm Angel City Tori’s Reef Red Slave

0.08 ± 0.00 0.08 ± 0.01 0.08 ± 0.00 0.19 ± 0.03 0.75 ± 0.61 0.20 ± 0.20 0.52 ± 0.49 0.70 ± 0.55 0.13 ± 0.08 0.25 ± 0.21 0.15 ± 0.05

0.08 ± 0.03 0.38 ± 0.10 0.21 ± 0.06 0.46 ± 0.23 0.25 ± 0.13 0.15 ± 0.01 0.14 ± 0.03 0.22 ± 0.05 0.14 ± 0.05 0.08 ± 0.03 0.05 ± 0.05

0.08 ± 0.03 0.43 ± 0.15 0.24 ± 0.10 0.66 ± 0.23 1.02 ± 0.72 0.30 ± 0.24 0.66 ± 0.50 0.93 ± 0.50 0.23 ± 0.18 0.32 ± 0.24 0.20 ± 0.05

0.037 ± 0.003 0.037 ± 0.003 0.043 ± 0.006 0.050 ± 0.010 0.057 ± 0.012 0.040 ± 0.000 0.080 ± 0.030 0.038 ± 0.003 0.035 ± 0.000 0.035 ± 0.000 0.040 ± 0.000

(2) (2)

2013 May Playa Funchi Karpata Cliff Front Porch Ebo’s Special Playa Lechi South Bay 18th Palm Angel City Tori’s Reef Red Slave

0.18 ± 0.06 0.21 ± 0.05 0.25 ± 0.14 0.19 ± 0.13 (1) 0.23 ± 0.11 0.31 ± 0.08 0.23 ± 0.09 0.77 ± 0.69 0.48 ± 0.20 0.24 ± 0.03 0.25 ± 0.03

0.39 ± 0.07 0.28 ± 0.03 0.43 ± 0.09 0.49 ± 0.14 0.47 ± 0.38 0.44 ± 0.04 0.43 ± 0.10 0.63 ± 0.45 0.82 ± 0.43 0.34 ± 0.05 0.45 ± 0.04

0.55 ± 0.11 0.49 ± 0.05 0.68 ± 0.16 0.68 ± 0.28 0.68 ± 0.48 0.76 ± 0.12 0.66 ± 0.18 1.39 ± 1.14 1.30 ± 0.62 0.57 ± 0.08 0.70 ± 0.06

0.070 ± 0.010 0.037 ± 0.003 (2) 0.083 ± 0.006 0.070 ± 0.010 0.063 ± 0.012 0.083 ± 0.012 0.057 ± 0.012 0.093 ± 0.059 0.057 ± 0.015 0.045 ± 0.009 (1) 0.063 ± 0.006

2011 November Playa Funchi Karpata Cliff Front Porch Ebo’s special Playa Lechi South Bay 18th Palm Angel City Tori’s Reef Red Slave

(3) (2) (2)

(2) (1) (2) (1)

between seasons nor regions was found. Chlorophyll-a concentrations significantly increased over time (p < 0.001), but ecological consequences of this slight increase are not studied (Fig. 2). More data in time are needed to confirm a structural trend. Our 834 measurements of water quality parameters for the waters of Bonaire are the first peer reviewed presented data to use in broader research. Statistical differences are based on this dataset which is limited in data points in time. Data points below limit of quantification additionally distorts its statistical power. To take into account yearly and seasonal differences additional data points in time are needed to improve statistical power and magnitude if the observations. Besides statistical significance, the ecological relevance of the dataset is considered to be significant for the discussion on water quality of Bonaire’s reef. These data show that occasionally some of the applied environmental threshold values for eutrophication are exceeded. This indicates slight but no year-round eutrophic conditions for the marine waters of Bonaire. For all nutrient indica-

Total P (lmol P–PO4/l)

Total N (lmol N–NO3/l)

(3) (3) (1)

(3) (2) (2) (2) (2) (2)

(1) (3) (3)

0.10 0.11 0.18 0.21 0.10 0.14 0.10 0.10 0.12 0.1 0.14

(1)

Chlorophyll-a (lg/l)

14.55 10.4 4.25 6.7 7.91 6.15 1.24 (1) 21.06 9.32 6.87 6.31

0.069 ± 0.004 0.064 ± 0.040 0.111 ± 0.008 0.103 ± 0.001 0.121 ± 0.060 0.133 ± 0.012 0.091 ± 0.008 0.130 ± 0.020 0.080 ± 0.009 0.087 ± 0.005 0.210 ± 0.184

0.21 0.19 0.19 0.15 0.17 0.13 0.14 0.19 0.15 0.14 0.15

9.64 8.09 9.25 4.8 6.33 5.86 5.69 9.04 7.19 6.6 6.86

0.131 ± 0.014 0.173 ± 0.003 0.133 ± 0.003 0.116 ± 0.015 0.104 ± 0.011 0.223 ± 0.029 0.137 ± 0.033 0.165 ± 0.024 0.130 ± 0.008 0.109 ± 0.017 0.128 ± 0.018

0.14 0.61 0.3 0.24 0.2 0.44 0.21 0.18 0.55 0.14 0.55

4.7 5.15 5.39 7 4.82 10.01 8.38 10 9.44 5.14 6.12

0.129 ± 0.009 0.090 ± 0.022 0.175 ± 0.011 0.187 ± 0.020 0.150 ± 0.016 0.160 ± 0.041 0.106 ± 0.009 0.127 ± 0.023 0.097 ± 0.014 0.151 ± 0.006 0.139 ± 0.005

(1) (1) (1)

tors, it is important to realize that the applied threshold value represents a generic cautionary risk level for eutrophication. Vermeij et al. (2010) reported that nutrient enrichment gave turf algae an overall competitive advantage over corals in an experimental research conducted at Curacao. As nutrients are in a constant flux, and allocated quickly into primary producers such as turf algae, macro algae, phytoplankton (Szmant, 2002; Fabricius, 2005) our preliminary conclusion should be interpreted with care. Additional measurements on both benthic coverage and internal nutrient concentration in tissues of turfs, algae and corals is needed to further assess actual eutrophic status of Bonaire. Phosphorus is of much interest in this study, as we speculate that concentrations increase over time. The observed increase in chlorophyll-a concentrations could be a consequence. Considering the recent reset of the threshold level for chlorophyll-a to 0.2 lg/l by Bell et al. (2013) and the values found during this study, chlorophyll-a could be taken as an easy to measure early warning indicator for eutrophication in Bonaire.

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Playa Funchi

12°18′

Latitude (N)

12°15′

DIN NH4 NO3 tN

PO4 tP

Chl

Karpata

DIN NH4 NO3 tN

PO4 tP

Chl

Cliff

2011 Nov

2011 Nov

2011 Nov

2012 May

2012 May

2012 May

2012 Nov

2012 Nov

2012 Nov

2013 May

2013 May

2013 May

Ebo’s Special

DIN NH4 NO3 tN

PO4 tP

Chl

Front Porch

2011 Nov

2011 Nov

2012 May

2012 May

2012 Nov

2012 Nov

2013 May

2013 May

DIN NH4 NO3 tN

PO4 tP

Chl

DIN NH4 NO3 tN

PO4 tP

Chl

DIN NH4 NO3 tN

PO4 tP

Chl

DIN NH4 NO3 tN

PO4 tP

Chl

12°12′ South Bay

DIN NH4 NO3 tN

PO4 tP

Chl

Playa Lechi

Bonaire

2011 Nov

2011 Nov

2012 May

12°09′

2012 May

2012 Nov

2012 Nov

2013 May

Angel City

DIN NH4 NO3 tN

PO4 tP

2013 May

Chl

2011 Nov Tori’s Reef

12°06′

DIN NH4 NO3 tN

PO4 tP

Chl

2012 May

18th Palm

2011 Nov

2012 Nov

2011 Nov

2012 May

2013 May

2012 May

2012 Nov

2012 Nov

2013 May

Red Slave

12°03′

DIN NH4 NO3 tN

PO4 tP

2013 May

Chl

2011 Nov 2012 May 2012 Nov

68°36′

>0 X

2013 May

Caribbean Sea 68°30′

68°24′

68°18′

68°12′

>0.9 X

>1 X

68°06′

>1.5 X threshold

NA

68°00′

Longitude (W) Fig. 3. Graphical summary of where and when environmental threshold concentrations for eutrophic risk indicators are exceeded. Indicators applied are DIN, NH4, NO3, total N, PO4, total P and chlorophyll-a. Factors (0; 0,9; 1; 1,5) represent how near (ratio) the threshold levels for each indicator are met and express the relative risk in sequential grey scales. X = data not available. The threshold values are presented in Table 4.

Measured concentrations depend largely on the timing of sampling in relation to diurnal dynamics in water quality. The latter depends on e.g. tidal regime, rain events, and local upwelling. Besides abiotic factors, biological factors steer temporal dynamics as well. via e.g. primary production steered by variable sun or cloud cover during a day. The magnitude of the variation and the implications for our data is unknown. Literature on temporal changes in water quality mostly reflect seasonal or yearly changes. Gast et al. (1999) reported temporal changes in water quality within a period of a week. Concentrations varied in that study with a factor 2. Diurnal variation in our study could have been very large as well. We do not know how large the 24 h dynamics were during our sampling events. However, our dataset includes four data points in time in which a structural increasing trend across almost all locations is observed for phosphorus, nitrate and chlorophyll. Although these 4 data points have limited statistical power, our hypothesis is that any temporal 24 h- fluctuations were dominated by observed longer term changes. Additional measurements have to be performed to strengthen this hypotheses. Besides temporal variation, spatial variation in water quality was observed as well. Specifically in the urban zone and the southern part of Bonaire (Playa Lechi, 18th Palm, Angel City, Tori’s Reef, Red Slave) threshold value of DIN is occasionally exceeded. At these locations highest concentrations for phosphate and chlorophyll a were observed. These observations are consistent with local nutrient pressures arising from the main urban and industrial zones of the island. The higher values near the urban area are most likely to be a result of sewage runoff due to a lack of a sewerage system (till 2014 at least). Gardening practices where raw sewage is used to watering plants adjacent to the sea might also be a contributing factor. Furthermore, significant salt flats works are located in the southern part of the island. These salt flat are responsible for organic matter build up through bacteria and shrimps population growing. Due to the karst characteristics of Bonaire, organic and nutrient enriched water can enter the reef in nearby locations such as Red Slave, Tori’s Reef and Angel City.

In nutrient limited environments, nutrients are assimilated quickly and which occurs most likely locally. D’Angelo and Wiedenmann (2014) observed spatial indirect effects of eutrophication. The prevailing current at Bonaire’s coast is from south to north. Based on (D’Angelo and Wiedenmann, 2014) it cannot be excluded that via pelagic algal growth in the south (e.g. Angel City) indirect eutrophic effects can be observed at locations laying north. The coral reef is not only valuable due to its intrinsic characteristics. The reef a crucial contributor to the economic value of Bonaire through fisheries and dive tourism (Cado van der Lely et al., 2013). Safeguarding the ecosystem is crucial for Bonaires livelihood. Based on the latest published threshold levels for eutrophication in coral reef environments, this preliminary evaluation of water quality for the coastal zone of Bonaire suggests (near-) eutrophic conditions. This is both a cause for concern and action to prevent (further) deterioration of the reef ecosystem. To address the nutrient problem, a sewage treatment plant, planned to be operational in 2014, will connect enterprises, hotels and households in the urban area around Kralendijk. These collected baseline data will assist the evaluation the effectiveness of the sewage treatment plant in future. In addition to measures to address sewage nutrient contamination, we recommend that our baseline work should be expanded into a monitoring program that integrates structural monitoring of water quality with monitoring of benthic coverage and ecosystem functioning. Acknowledgments Principal funding was provided by the Ministry of Infrastructure and the Environment under project number 4305202701 (Diana Slijkerman, principal investigator). We thank Dr. R. Peachy and the technical staff of CIEE Bonaire, for general assistance and use of their laboratory facilities, Geert den Hartog (RWS meetdienst Zeeland) and Elsmarie Beukenboom (Stinapa) for technical assistance in the field, Marco Houtekamer (NIOZ) for nutrient analysis, Erika Koelemij (IMARES) for chlorophyll analysis and Fleur van Duyl (NIOZ), Edwin Foekema, Dolfi Debrot (IMARES) and Boris Teunis (Ministry) for feedback on preliminary versions of this

D.M.E. Slijkerman et al. / Marine Pollution Bulletin 86 (2014) 523–529

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