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Response of benthos to ocean outfall discharges: does a general pattern exist?
Marine Pollution Bulletin 101 (2015) 174–181
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Response of benthos to ocean outfall discharges: does a general pattern exist? A. Puente a,⁎, R.J. Diaz b a b
Environmental Hydraulics Institute “IH Cantabria”, Universidad de Cantabria, C/Isabel Torres N° 15, Parque Científico y Tecnológico de Cantabria, 39011, Spain College of William and Mary, Virginia Institute of Marine Science, Route 1208 Great Road, Gloucester Pt., VA 23062, USA
a r t i c l e
i n f o
Article history: Received 3 September 2015 Received in revised form 28 October 2015 Accepted 1 November 2015 Available online 12 November 2015 Keywords: Outfall Benthos Macroinvertebrates Sewage
a b s t r a c t We assessed the effects of 40 ocean outfalls on adjacent macrobenthic invertebrates. Data were obtained from a review of gray and peer-review literature. Different parameters describing the outfall characteristics were compiled (length, maximum depth, treatment level, flow and organic matter mass discharged). Exposure to wave action was represented by significant wave height. The magnitude of the effect was categorized in three impact levels and classified considering different ecological indicators. A theoretical predictive model was formulated in which the lower the organic matter and the higher the energy of the system, the lower the benthic impact. The main conclusion was that the general pattern of the succession of benthic communities brought about by ocean outfalls fits the model of Pearson–Rosenberg but with some deviations i) the probability of a significant impact is much lower, ii) not all the successional stages occur and, iii) the magnitude of the changes are usually lower. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction As coastal development and human population increase so does the number of sewage treatment plants and the disposal of effluents to the sea. This level of discharge raises concerns for ecological health, loss of species and habitat quality for benthic and pelagic organisms; and for human health, fecal contamination of bathing waters and harvested fish and shellfish (WHO (World Health Organization), 2003). In this situation, the discharge of adequately treated wastewater through well designed ocean outfalls appears a good option to improve the sanitation system of highly populated coastal areas (Juanes et al., 2004a; Roberts et al., 2010). There are well over 500 coastal or ocean outfalls discharging effluents to the sea (IAHR/IWA, 2014). These outfalls deliver a range of dissolved and solid constituents to near shore areas. For the benthos, constituents of primary concern are particulate organic matter (POM) and contaminants absorbed to particles. Freshwater, nutrients, and dissolved organic matter (DOM) are of greater concern for pelagic habitats. The latter two become a concern for the benthos if they support phytoplankton blooms that will deliver additional POM to the bottom. The main physical processes that govern the mixing and evolution of wastewater in the ocean are turbulent dispersion, transport (advection and diffusion) and resuspension (Kundu and Cohen, 1990; Roberts et al., 2010). Near field mixing (10 to 1000 m) depends on the discharge parameters (buoyancy and momentum) and are under the control of the designer, whereas far field mixing (100 to 10 km) processes are ⁎ Corresponding author.
http://dx.doi.org/10.1016/j.marpolbul.2015.11.002 0025-326X/© 2015 Elsevier Ltd. All rights reserved.
mainly dependent on transport by ocean currents and diffusion by oceanic turbulence (Roberts et al., 2010). Large scale flushing and dilution, and metabolic processes remove contaminants and reduce long-term accumulation of pollutants (Roberts et al., 2010). The impact of any one constituent on the surrounding pelagic and benthic habitats will depend on several factors such as: i) the effluent discharge flow rate and total mass and composition of constituents, ii) flux characteristics of the outfall (length of diffuser, orientation, ports number, jet diameter, jet spacing), iii) the geometry (seafloor topography, depth) and dynamic conditions of the aquatic receiving system (density stratification, sedimentation patterns, velocities of the bottom currents) (Jirka and Harleman, 1979). In high energy environments all constituents will be broadly dispersed with a minor chance of concentrating, but in low energy environments there is the potential for particulates and substances adsorbed to particles to accumulate near the outfall (Roberts et al., 2010; Gómez et al., 2014). Benthic organisms have long been used to monitor the effects of organic enrichment and other types of pollution, because their sedentary life-histories and longevity make them integrators of impacts (Warwick, 1993; Dauer et al., 2000; Diaz et al., 2004). Much of what we know about the effects of sewage discharges into enclosed inshore systems such as rivers, estuaries, and fjords cannot be directly applied to ocean discharges as hydrodynamics are so different. In lower energy, accumulating bottoms, the paradigm developed by Pearson and Rosenberg (1978) that describes the response of benthos to a gradient of organic enrichment has been applied in many situations (Boesch and Rosenberg, 1981; Dauer, 1993; Diaz and Rosenberg, 1995; Borja et al., 2006). In higher energy, dispersive bottoms, this paradigm does
A. Puente, R.J. Diaz / Marine Pollution Bulletin 101 (2015) 174–181
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Table 1 Outfalls considered in this study.
Country
Australia
Canada
Depth (m)
Wave Size height Effect (m3.day−1) (HSm) (m)
Pretreated 1.20
15.00
55,000
2.95
Primary Primary Primary
2.22 3.62 3.66
63.00 60–80 60–65
126,000 498,000 307,000
1.30 1.30 1.30
−123.423195°
Untreated
1.70
60.00
43,754
1.82
49.201242°
−123.300556°
Primary
7.50
Penco (Concepción)
−36.715745°
−73.001485°
Tomé (Concepción)
−36.630104°
−72.970048°
Saronikos Gulf (Athens)
37.940659°
23.610754°
Untreated
0.30
30.00
600,000
0.35
Grado (Gulf of Trieste) Barcola (Gulf of Trieste) Lignano (Gulf of Trieste) Porto Buso (Gulf of Trieste) Trieste/Servola (Gulf of Trieste) Gisborne Hastings
45.634603° 45.689011° 45.616935° 45.650573°
13.408020° 13.726413° 13.190410° 13.274827°
Primary Primary Secondary Secondary
4.80 0.96 7.50 7.60
10.00 19.00 16.00 15.00
39,400 6000 60,500 153,800
0.37 0.15 0.38 0.27
Primary
6.5–7.5 23.00
81,000
0.15
Near field
Cibic et al. (2008)
Untreated Primary
2.90 1.80
24,192 69,120
2.07 1.59
Near field Near field
Roper et al. (1989) Roper et al. (1989) Silva et al. (2004); Sampaio et al. (2011) CSIR (2012) CSIR (2012) Del-Pilar-Ruso et al. (2010); De-la-Ossa-Carretero et al. (2012a, b)
Site
Latitude
Longitude
Black Rock (Victoria)
−38.294436°
144.427970°
Bondi (Sydney) Malabar (Sydney) North Head (Sydney)
−33.881170° −34.140048° −33.826000°
151.308987° 151.170736° 151.339782°
Mcaulay Point (Victoria)
48.402252°
Strait of Georgia
Treatment
Lenght (km)
89.00 567,000
1.82
Pretreated 1.30
22.00 12,000
2.46
Pretreated 1.20
25.00 15,000
2.46
Chile
Greece
Italy
New Zeland
46.652325°
13.682926°
−38.725660° −39.705238°
178.089877° 177.411011°
13.00 17.00
Ashton and Richardson (1995) Near field Otway (1995) Near field Otway (1995) Near field Otway, (1995) Chapman et al. (1996); Near field CRD (Capital Regional District) (2008) Far field Burd et al. (2012) Leppe and Padilla No significant (1999)Roberts et al. (2010) Leppe and Padilla (1999); No significant Roberts et al. (2010) Simboura et al. (1995); Far field Makra et al. (2001); Simboura et al. (2014) Near field Solis-Weiss et al., (2007) Near field Solis-Weiss et al. (2007) No significant Solis-Weiss et al. (2007) Near field Solis-Weiss et al. (2007) No significant
38.670154°
−9.450399°
Pretreated 2.75
40.00
138,000
1.45
Near field
−29.916786° −29.998606°
31.081357° 30.994056°
Pretreated 4.2 Pretreated 3.2
60 50
135,000 230,000
1.73 1.73
Near field Far field
Alcossebre (Castellón)
40.242009°
0.308662°
Pretreated 1.89
14.00
1800
0.54
Far field
Algeciras (External Saladillo Harbour)
36.115631°
−5.425414°
Untreated
Unknown 22,580
0.63
Near field
Benicarló (Castellón)
40.413154°
0.461644°
Pretreated 2.14
14.58
0.62
Canet (Valencia)
39.669870°
−0.186770°
Pretreated N1000
13.6–16.8 4232
0.49
Gandía (Valencia)
39.006918°
−0.129063°
Secondary 1.90
16.84
45,280
0.60
Oliva (Valencia)
38.933864°
−0.099581°556 Secondary
15.50
3947
0.60
Portugal
Lisbon. Estoril
South Africa
Durban (Central) Durban (Southern)
1.00
16,754
Spain
Peñíscola (Castellón)
40.348114°
0.428319°
Pretreated 2.00
15.50
9500
0.62
Sant Adriá del Besós
41.409111°
2.238113°
Primary
10–25
400,000
0.49
Santander (Cantabria)
43.501710°
−3.884236°
Secondary 2.80
45.00
75,000
2.11
Torreblanca (Castellón)
40.189769°
0.254367°
Secondary 2.18
14.00
1442
0.52
San Sebastián-Pasajes
43.347391°
−1.945997°
Vinaroz (castellón)
40.461483°
0.507634°
56.026233°
−3.182764°
42.335789° 21.280714° 21.454455° 21.278893° 21.424799°
United Edinburgh Kingdom Boston (Massachusetts) O'ahu, Honouliuli (Hawai) O'ahu, Mokapu (Hawai) O'ahu, Sand Island (Hawai) O'ahu, Waianae (Hawai) USA Orange. San Pedro Shelf (California) San Diego. Point Loma (California) Woods Hole (Massachusetts)
Untreated
0.60
Reference
Estacio et al. (1997)
Del-Pilar-Ruso et al. (2010); De-la-Ossa-Carretero et al. (2012a, 2012b) Del-Pilar-Ruso et al. Near field (2010); Del-Pilar-Ruso et al. (2010); Near field De-la-Ossa-Carretero et al. (2012b) Del-Pilar-Ruso et al. (2010); Near field De-la-Ossa-Carretero et al. (2012b) De-la-Ossa-Carretero Near field et al. (2012a, 2012b) Far field Cardell et al. (1999) Juanes et al. (2004b); No significant Echavarri-Erasun et al. (2007) Del-Pilar-Ruso et al. (2010); No significant De-la-Ossa-Carretero et al. 2012a, 2012b) Far field Franco et al. (2004) De-la-Ossa-Carretero Far field et al. (2012a, 2012b) Far field
1.20
50.00
55,000
1.72
Pretreated 2.49
15.81
7600
0.55
Primary
1.50
30.00
134,587
1.31
No significant Read et al. (1983)
−70.743562° −157.990229° −157.693227° −157.897718° −158.197063°
Secondary Primary Secondary Primary Secondary
16.00 2.00 4.00 2.74 1.50
32.00 61.00 32.00 72.50 32.50
375,000 85,536 51,840 256,608 13,824
0.82 2.30 2.11 2.30 2.32
No significant No significant No significant No significant No significant
33.566966°
−118.004541°
Secondary 8.00
60.00
852,000
1.51
Near field
32.663890°
−117.332071°
41.522697°
−70.673944°
Maciolek et al. (2008) Shuai et al. (2014) Shuai et al. (2014) Shuai et al. (2014) Shuai et al. (2014) Maurer et al. (1998, 2007); Diener et al. (1995)
Primary
7.24
98.00
681,374
1.44
Near field
City of San Diego (2009)
Untreated
0.18
14.00
1175
0.82
Near field
Smith et al. (1973)
176
A. Puente, R.J. Diaz / Marine Pollution Bulletin 101 (2015) 174–181
not adequately describe the response of benthos (Maurer, 1993; Echavarri-Erasun et al., 2010). The aim of this paper is to assess the effects of ocean outfalls on adjacent macrobenthic invertebrates, and to test the influence of the outfall characteristics and the physical processes on the magnitude of impacts. 2. Methods Data were obtained from gray and peer-review published literature. Search key words were: sewage, outfall, ocean, benthos (or benthic or macrobenth*). We compiled information from 40 ocean outfalls in 11 countries (Australia, 4; Canada, 2; Chile, 2; Greece, 1; Italy, 4; New Zealand, 2; Portugal, 1; South Africa, 2; Spain, 13; United Kingdom, 1; USA, 8) (Table 1). Each outfall was characterized according to length (m), maximum depth (m), treatment level (untreated, pretreated, primary treatment, secondary or better treatment), flow (m3.day−1) and organic matter mass discharged (kg.day−1). Organic matter was estimated using BOD5 and was calculated from the combination of flow and concentration of effluent considering the treatment level received (untreated: 400 mg/l; pretreated: 300 mg/l; primary: 180 mg/l; secondary or better: 25 mg/l) (WRC (Water Research Center), 1990). Exposure to wave action was used as a proxy for energy of the system where the discharge occurred. Wave action was represented by significant wave height (Hs), mean height (Hsm) and 95-percentile (Hsp95), based on a global wave dataset simulated with the model WaveWatch III and driven by NCEP/NCAR reanalysis of winds and ice fields (Global Ocean Waves, GOW) (Reguero et al., 2012). The magnitude of the impact of the outfall on macrobenthic assemblages was based on the results and conclusions of studies reviewed. The magnitude was categorized according to the detection or not of significant effects and the distance at which significant effects was detected (no effect or very low; significant effect in the near field, b 500 m; significant effect in the far field, N500 m). In addition, the type of ecological effects reported was classified considering the following indicators: i) Effect on community structure parameters (richness, diversity, abundance, biomass, coverage). ii) Changes in the proportion of opportunistic-sensitive species or trophic groups. iii) Changes in the community structure, detected by the multivariate analyses.
To synthesize the types of ocean outfalls and environmental conditions, we applied a combination of two classification techniques, SelfOrganizing Maps (SOM) (Kohonen, 2001) and K-means algorithm
(Hastie et al., 2001). The SOM is a classification method included in artificial neural networks (ANNs) that detects patterns or classes in a set of data, preserving the neighboring relations. This means that similar clusters in the multidimensional space are located together on a 2D grid that allows the data to be intuitively visualized. The starting point of this technique is a data sample in which N is the total number of data points to be classified. The ANN is “trained” using an iterative learning algorithm. The process includes a self-organizing neighborhood mechanism, so neighboring clusters of the winning reference vector in the 2D lattice space are also adapted toward the sample vector, thus projecting the topological neighborhood relationships of the highdimensional data space onto the lattice. In this study, a lattice of 30 neurons (5 × 6) was chosen as an optimum solution. The input variables of the analyses were outfall size and organic matter discharge, as descriptors of outfall characteristics, and depth and wave height (mean, percentile 95), as indicators of the physical environment. The number of groups obtained with the application of the SOM was then clustered by a K-means algorithm. The number of k-mean groups was justified according to the minimum Davies Bouldin Index (DBI) for a solution with low variance within clusters and high variance between clusters (Negnevitsky, 2002). SOM analyses were conducted using Matlab 7.7 and the SOM coding solution based on SOM Toolbox for Matlab 5 (Vesanto et al., 2000).
3. Results Our review of the literature found that for ocean outfalls, most of the studies concluded that there was an absence of any kind of impacts or classified them as not significant (30%) or the effect was restricted to the plume mixing zone or the near field within 500 m of the outfall (50%). The main types of effects reported were changes in the proportion of opportunistic/sensitive species (89%), a decrease in richness and/or diversity (61%), an increase in total abundance (54%), and changes in community structure (39%) (Fig. 1). Almost half of the ocean outfalls included in the study discharged wastewater without previous treatment or only pretreated, and most of them (82%) were reported to have a significant effect, both near field (47%) and far field (35%) (Fig. 2). The three levels of impacts considered were detected in treated discharges, although none of the outfalls with secondary treatment showed an impact beyond the near field. With respect to flow and mass loading it stands out that all of the biggest outfalls were reported to have a significant effect, although some of those with very high organic matter input did not seem to have any kind of impacts. On the other hand, even some of the smallest outfalls showed a significant impact in the far field. Impacts were detected in a wide range of depths, although a high proportion of
Fig. 1. Type of effects reported in the papers reviewed (number of outfalls).
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those discharging in deep bottoms had a significant impact, both near and far field. Finally, it should be highlighted that none of the outfalls located in very exposed environments showed a deleterious effect in the far field. Patterns emerged when gradients in both organic matter inputs from the outfalls and the energy of the environment were plotted together (Fig. 3). No significant effects were observed when the outfalls fulfill the following characteristics i) low organic matter load (b 10,000 kg.day−1), even in sheltered environments (Hsm b 1 m), ii) medium to very high organic matter load (N10,000 kg.day−1) located in exposed or very exposed sites (HsmN 2 m). Outfalls with reported significant effects even beyond the mixing zone exhibit the following conditions i) low organic matter load, located in sheltered environments; ii) high organic matter load (N 20,000–30,000 kg.day−1) located in semiexposed
177
environments (Hsm N 1–1.5 m) and, iii) very high organic matter load (N 30,000 kg.day−1), except in very exposed environments (Hsm N 2 m). A mixture of secenarios appeared when the deleterious effect was restricted to the near field, although most of the cases were outfalls with low–moderate organic matter located in sheltered environments, or with high–very high matter load discharging in semiexposed sites. From the SOM and K-means analyses, the minimum Davies Bouldin index obtained determined that 4 was the optimal number of groups for the K-means analysis. The range of variation of each variable was represented on the trained SOM. In addition, with the aim of facilitating the interpretation of the results we have represented the magnitude of the effect considering a gradient from 1 to 3 (1 = no significant effect, 2 = near field effect, 3 = far field effect). Based on these results it was possible to observe how the outfalls were grouped according to their
Fig. 2. Number of outfalls reported as not having deleterious effects and those with demonstrated significant effect (near and far field) in different categories of the factors analyzed.
178
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intrinsic characteristic and the physical energy in the surrounding environment (Fig. 4). The main characteristics of the four groups identified by K-means analysis are: - Group A. It comprises discharges with low to moderate organic matter input (from 12,000 to 75,000 m3.day−1; 346 to 22,000 kg.day−1) and all of them located in very exposed environments except Hastings (Hsm = 1.59 m) and San Sebastián-Pasajes (Hsm = 1.72 m). The level of treatment ranges from untreated to secondary. Most of the outfalls in this group did not show a significant effect (Black Rock, O'ahu-Waianae, O'ahu- Mokapu, Penco, Santander, Tomé), although two of them were reported to have an effect in the near field (Gisborne and Hastings) and one even in the far field (San Sebastián-Pasajes). - Group B. It includes outfalls with medium to high organic matter discharge (from 437,754 to 567,000 m3.day−1; 15,396 to 102,060 kg.day-1). All of them were untreated or with primary treatment. The environment was exposed (Hsm N 1.7 m) or very exposed (those of Hawai Hsm = 2.3), and only the Lisbon outfall was in a somewhat sheltered environment (Hsm = 1.45 m). In general, these group clusters discharges with moderate impacts in the near field (Bondi, Durban (Central), Durban (Southern), Lisbon, Mcaulay Point, North Head), although two of them were considered to have a very low impact (O'ahu-Honouliuli, O'ahu-Sand Island) and one a significant impact even in the far field (Strait of Georgia). - Group C. This group includes outfalls with low to moderate organic matter input (from 1175 to 375,000 m3.day−1; 36 to 24,226 kg.day−1) located all of them in sheltered environments (Hsm b 0.8) or semiexposed environments (Edinburgh, Hsm = 1.31). The magnitude of their impacts was in most of the cases moderate (Algeciras, Barcola, Canet, Gandía, Grado, Oliva, Peñíscola, Porto Busto, Trieste/ Servola, Woods Hole), but some of them were considered not to have an impact (Boston, Edinburgh, Lignano, Torreblanca); and three had a deleterious effect even beyond the mixing zone (Alcossebre, Benicarló, Vinaroz). - Group D. It contains outfalls with high organic matter input (from 400,000 to 852,000 m3.day−1; 21,300 to 240,000 kg.day−1) and located in semiexposed environments (Hsm N 1.5), except Saronikos Gulf
where Hsm = 0.35 m. This cluster includes the outfalls with the highest impacts (Sant Adriá del Besós, Saronikos Gulf), although in those of Malabar, Orange and San Diego the impact seems to be restricted to the mixing zone.
4. Discussion Our main conclusion is that impacts linked to ocean outfall discharges are generally lower than expected relative to impacts from embayment outfalls. The evolution of sewage treatment and relocation of outfalls from within Boston Harbor, USA, to an ocean outfall provide a good example (Signell et al., 2000; Diaz et al., 2008). Most significant ocean outfall effects detected are limited to the near field (b 500 m from the outfall). These results support the statement of Roberts et al. (2010) that properly designed ocean outfalls do not cause significant ecological impacts. The widely applied model described by Pearson and Rosenberg (1978) for response of benthos to organic matter gradients needs to be revised for coastal areas. The balance between the total organic mass loading to bottom waters and the energy of the system seem to be the key factors in determining benthic impacts. Thus, when the amount of organic matter is low, the environment is usually able to assimilate the contamination, even in sheltered environments and untreated effluents. Moreover, even big outfalls do not show a measurable deleterious effect beyond the near field, when they are located in exposed environments or appropriately treated. The combination of a range of organic matter inputs with a gradient in the exposure to waves, from sheltered to very exposed environments, results in a wide range of magnitude and spatial extent of impacts. The availability of oxygen in the sediments is involved in the variety of responses of benthic communities in the studies reviewed. The depletion of oxygen by decomposition of organic matter is perhaps the most important factor in the structuring of benthic communities and the ecosystem functioning in polluted sites (Pearson and Rosenberg, 1978; Diaz and Rosenberg, 1995, 2008; Conley et al., 2009). However, in the open sea the high initial dilution (at least 100:1), the intense mixing and the large surface area available for re-aeration makes it difficult to
Fig. 3. Number of outfalls reported as not having deleterious effects and those with demonstrated significant effect (near and far field) in different scenarios combining the rate of organic matter loading (Low: b10,000 kg.day−1; Medium: N10,000–20,000 kg.day−1; High: N20,000–30,000 kg.day−1; Very high: N30 000 kg.day−1) and the mean significant wave height (Sheltered: b1 m; Semiexposed: N1–1.5 m; Exposed: N1.5–2 m; Very exposed: N2 m).
A. Puente, R.J. Diaz / Marine Pollution Bulletin 101 (2015) 174–181
generate hypoxia, especially in those environments with high wave height or currents (Roberts et al., 2010; Conley et al., 2009). In addition, despite of the ecological importance of hypoxia most marine invertebrates are not significantly affected until very low concentration (Diaz and Rosenberg, 1995). Several authors have established the thresholds for sublethal effects between 2.0 and 2.8 mg O2 l−1 (Tyson
179
and Pearson, 1991; Diaz and Rosenberg, 1995; Vaquer-Sunyer and Duarte, 2008), a concentration that is typically reached under very high organic input and poor flushing conditions (estuaries, enclosed seas, stratified water bodies). The relationship between water renewal and organic enrichment has already been reported before for coastal areas or even estuaries.
Fig. 4. A) Gradient analysis of each physical variable on the trained SOM, with visualization in a shading scale (light = high values, dark = low values). B) K-mean results on the SOM plane, (A, B, C, D denotes the four groups identified). C) Visualization of the average magnitude of the impact in each neuron, with visualization shading (blue = low impact, red = high impact). The outfalls included in each neuron are indicated.
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For example, Roper et al. (1989) found little effect in the macroinvertebrate assemblages near a sewage outfall discharging 69,000 m3.d−1 into an estuary, in Tauranga Harbour, and concluded that is probably related to the high water velocities over the outfall diffuser (up to 0.5 m s−1). Pearson and Rosenberg (1978) in describing macrobenthic succession along a gradient of organic enrichment, held that oxygen would not be a limiting factor in well-flushed bottoms and an increase in benthic biomass could be expected, stimulated by excess organic matter in the ecosystem. Therefore, based on these premises and supported by the SOM trained map from our analysis we formulated a “model” in which the lower the organic matter and the higher the energy of the system, the lower the impact. However, we found some exceptions to this general statement in the papers reviewed. For example, lower impacts would be expected in the case of Gisborne, Hastings and San Sebastián-Pasajes outfalls (Group A), because all of them are quite small and located in exposed environments. Nonetheless, regarding the two New Zealand outfalls, Roper et al. (1989) remarked that no grossly polluted zone devoid of macrofauna was found and the polluted zone was limited to within 200 m of the diffusers. In addition, it should be taking into account that both Gisborne and San Sebastián are untreated, and the later discharge is close to a previously degraded area. The outfalls of O'ahu-San Island (Group B) were expected to have a higher impact considering their size, but the energy of the system likely minimized impacts. As well, Lignano and Torreblanca (Group C) did not show a significant impact despite the extremely sheltered conditions at their discharge points, which is related to their small size and secondary treatment. Finally, the outfalls of Orange, San Diego and Malabar (Group D) seem to have a less impact than expected according to the model for their size. Orange and San Diego are very long (N7 km) and deep (70 and 98, respectively) which increases dilution capacity. Malabar is shorter but the discharge point is also quite deep (60 m). Similarly, the outfall of Boston (Group D) did not show any deleterious effect, despite discharging a high organic matter load in a sheltered environment. This outfall has secondary treatment, is extremely long (16 km with 2 km of diffusers) and quite deep (32 m). These results emphasize the importance of the treatment and the outfall design in minimizing environmental impacts. In other cases, the unexpected results could be explained by an over or under estimation of the system's energy. For example, the wave height in the Strait of Georgia (Group B) may be lower than that estimated due its geomorphology, and the currents in Edinburgh (Group C) may contribute to increase the water renewal. Most of the significant impacts in the papers reviewed frequently described a decrease of richness and diversity, an increase in total abundance and changes in the composition of community, including a decline of abundances of sensitive species and increase of opportunistic species. Pearson and Rosenberg (1978) conclude that in polluted sites large individuals and long-lived equilibrium species are eliminated and population shifted toward younger individuals, and smaller and more short-lived species that possess opportunistic life histories. However, none of ocean outfalls evaluated reported a complete absence of fauna or a total dominance of opportunistic species as would correspond to the first stage of succession in a process of organic enrichment. Moreover, some of the assemblages colonizing the bottoms around the ocean discharges correspond to the ecotone point, with low richness and high abundance, or the transition zone, characterized by great fluctuations of the populations, described as successional stages between the peak of opportunistic species and the mature and stable community in the Pearson– Rosenberg paradigm. In summary, we can say that the general pattern of succession of benthic communities brought about by ocean outfalls fits the model of Pearson–Rosenberg with some deviations i) the probability of a significant impact is much lower ii) not all the successional stages occur and, iii) the magnitude of the changes are usually lower.
Acknowledgments The authors are very grateful to José A. Juanes (IH Cantabria) and Carlton Hunt (Battelle Memorial Institute) for organizing the International Workshop on Marine Outfalls Discharges (IWMOD 2010), which was the origin of this paper. We also thank to Elvira Ramos for her help in the statistical analysis.
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Response of benthos to ocean outfall discharges: does a general pattern exist? A. Puente a,⁎, R.J. Diaz b a b
Environmental Hydraulics Institute “IH Cantabria”, Universidad de Cantabria, C/Isabel Torres N° 15, Parque Científico y Tecnológico de Cantabria, 39011, Spain College of William and Mary, Virginia Institute of Marine Science, Route 1208 Great Road, Gloucester Pt., VA 23062, USA
a r t i c l e
i n f o
Article history: Received 3 September 2015 Received in revised form 28 October 2015 Accepted 1 November 2015 Available online 12 November 2015 Keywords: Outfall Benthos Macroinvertebrates Sewage
a b s t r a c t We assessed the effects of 40 ocean outfalls on adjacent macrobenthic invertebrates. Data were obtained from a review of gray and peer-review literature. Different parameters describing the outfall characteristics were compiled (length, maximum depth, treatment level, flow and organic matter mass discharged). Exposure to wave action was represented by significant wave height. The magnitude of the effect was categorized in three impact levels and classified considering different ecological indicators. A theoretical predictive model was formulated in which the lower the organic matter and the higher the energy of the system, the lower the benthic impact. The main conclusion was that the general pattern of the succession of benthic communities brought about by ocean outfalls fits the model of Pearson–Rosenberg but with some deviations i) the probability of a significant impact is much lower, ii) not all the successional stages occur and, iii) the magnitude of the changes are usually lower. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction As coastal development and human population increase so does the number of sewage treatment plants and the disposal of effluents to the sea. This level of discharge raises concerns for ecological health, loss of species and habitat quality for benthic and pelagic organisms; and for human health, fecal contamination of bathing waters and harvested fish and shellfish (WHO (World Health Organization), 2003). In this situation, the discharge of adequately treated wastewater through well designed ocean outfalls appears a good option to improve the sanitation system of highly populated coastal areas (Juanes et al., 2004a; Roberts et al., 2010). There are well over 500 coastal or ocean outfalls discharging effluents to the sea (IAHR/IWA, 2014). These outfalls deliver a range of dissolved and solid constituents to near shore areas. For the benthos, constituents of primary concern are particulate organic matter (POM) and contaminants absorbed to particles. Freshwater, nutrients, and dissolved organic matter (DOM) are of greater concern for pelagic habitats. The latter two become a concern for the benthos if they support phytoplankton blooms that will deliver additional POM to the bottom. The main physical processes that govern the mixing and evolution of wastewater in the ocean are turbulent dispersion, transport (advection and diffusion) and resuspension (Kundu and Cohen, 1990; Roberts et al., 2010). Near field mixing (10 to 1000 m) depends on the discharge parameters (buoyancy and momentum) and are under the control of the designer, whereas far field mixing (100 to 10 km) processes are ⁎ Corresponding author.
http://dx.doi.org/10.1016/j.marpolbul.2015.11.002 0025-326X/© 2015 Elsevier Ltd. All rights reserved.
mainly dependent on transport by ocean currents and diffusion by oceanic turbulence (Roberts et al., 2010). Large scale flushing and dilution, and metabolic processes remove contaminants and reduce long-term accumulation of pollutants (Roberts et al., 2010). The impact of any one constituent on the surrounding pelagic and benthic habitats will depend on several factors such as: i) the effluent discharge flow rate and total mass and composition of constituents, ii) flux characteristics of the outfall (length of diffuser, orientation, ports number, jet diameter, jet spacing), iii) the geometry (seafloor topography, depth) and dynamic conditions of the aquatic receiving system (density stratification, sedimentation patterns, velocities of the bottom currents) (Jirka and Harleman, 1979). In high energy environments all constituents will be broadly dispersed with a minor chance of concentrating, but in low energy environments there is the potential for particulates and substances adsorbed to particles to accumulate near the outfall (Roberts et al., 2010; Gómez et al., 2014). Benthic organisms have long been used to monitor the effects of organic enrichment and other types of pollution, because their sedentary life-histories and longevity make them integrators of impacts (Warwick, 1993; Dauer et al., 2000; Diaz et al., 2004). Much of what we know about the effects of sewage discharges into enclosed inshore systems such as rivers, estuaries, and fjords cannot be directly applied to ocean discharges as hydrodynamics are so different. In lower energy, accumulating bottoms, the paradigm developed by Pearson and Rosenberg (1978) that describes the response of benthos to a gradient of organic enrichment has been applied in many situations (Boesch and Rosenberg, 1981; Dauer, 1993; Diaz and Rosenberg, 1995; Borja et al., 2006). In higher energy, dispersive bottoms, this paradigm does
A. Puente, R.J. Diaz / Marine Pollution Bulletin 101 (2015) 174–181
175
Table 1 Outfalls considered in this study.
Country
Australia
Canada
Depth (m)
Wave Size height Effect (m3.day−1) (HSm) (m)
Pretreated 1.20
15.00
55,000
2.95
Primary Primary Primary
2.22 3.62 3.66
63.00 60–80 60–65
126,000 498,000 307,000
1.30 1.30 1.30
−123.423195°
Untreated
1.70
60.00
43,754
1.82
49.201242°
−123.300556°
Primary
7.50
Penco (Concepción)
−36.715745°
−73.001485°
Tomé (Concepción)
−36.630104°
−72.970048°
Saronikos Gulf (Athens)
37.940659°
23.610754°
Untreated
0.30
30.00
600,000
0.35
Grado (Gulf of Trieste) Barcola (Gulf of Trieste) Lignano (Gulf of Trieste) Porto Buso (Gulf of Trieste) Trieste/Servola (Gulf of Trieste) Gisborne Hastings
45.634603° 45.689011° 45.616935° 45.650573°
13.408020° 13.726413° 13.190410° 13.274827°
Primary Primary Secondary Secondary
4.80 0.96 7.50 7.60
10.00 19.00 16.00 15.00
39,400 6000 60,500 153,800
0.37 0.15 0.38 0.27
Primary
6.5–7.5 23.00
81,000
0.15
Near field
Cibic et al. (2008)
Untreated Primary
2.90 1.80
24,192 69,120
2.07 1.59
Near field Near field
Roper et al. (1989) Roper et al. (1989) Silva et al. (2004); Sampaio et al. (2011) CSIR (2012) CSIR (2012) Del-Pilar-Ruso et al. (2010); De-la-Ossa-Carretero et al. (2012a, b)
Site
Latitude
Longitude
Black Rock (Victoria)
−38.294436°
144.427970°
Bondi (Sydney) Malabar (Sydney) North Head (Sydney)
−33.881170° −34.140048° −33.826000°
151.308987° 151.170736° 151.339782°
Mcaulay Point (Victoria)
48.402252°
Strait of Georgia
Treatment
Lenght (km)
89.00 567,000
1.82
Pretreated 1.30
22.00 12,000
2.46
Pretreated 1.20
25.00 15,000
2.46
Chile
Greece
Italy
New Zeland
46.652325°
13.682926°
−38.725660° −39.705238°
178.089877° 177.411011°
13.00 17.00
Ashton and Richardson (1995) Near field Otway (1995) Near field Otway (1995) Near field Otway, (1995) Chapman et al. (1996); Near field CRD (Capital Regional District) (2008) Far field Burd et al. (2012) Leppe and Padilla No significant (1999)Roberts et al. (2010) Leppe and Padilla (1999); No significant Roberts et al. (2010) Simboura et al. (1995); Far field Makra et al. (2001); Simboura et al. (2014) Near field Solis-Weiss et al., (2007) Near field Solis-Weiss et al. (2007) No significant Solis-Weiss et al. (2007) Near field Solis-Weiss et al. (2007) No significant
38.670154°
−9.450399°
Pretreated 2.75
40.00
138,000
1.45
Near field
−29.916786° −29.998606°
31.081357° 30.994056°
Pretreated 4.2 Pretreated 3.2
60 50
135,000 230,000
1.73 1.73
Near field Far field
Alcossebre (Castellón)
40.242009°
0.308662°
Pretreated 1.89
14.00
1800
0.54
Far field
Algeciras (External Saladillo Harbour)
36.115631°
−5.425414°
Untreated
Unknown 22,580
0.63
Near field
Benicarló (Castellón)
40.413154°
0.461644°
Pretreated 2.14
14.58
0.62
Canet (Valencia)
39.669870°
−0.186770°
Pretreated N1000
13.6–16.8 4232
0.49
Gandía (Valencia)
39.006918°
−0.129063°
Secondary 1.90
16.84
45,280
0.60
Oliva (Valencia)
38.933864°
−0.099581°556 Secondary
15.50
3947
0.60
Portugal
Lisbon. Estoril
South Africa
Durban (Central) Durban (Southern)
1.00
16,754
Spain
Peñíscola (Castellón)
40.348114°
0.428319°
Pretreated 2.00
15.50
9500
0.62
Sant Adriá del Besós
41.409111°
2.238113°
Primary
10–25
400,000
0.49
Santander (Cantabria)
43.501710°
−3.884236°
Secondary 2.80
45.00
75,000
2.11
Torreblanca (Castellón)
40.189769°
0.254367°
Secondary 2.18
14.00
1442
0.52
San Sebastián-Pasajes
43.347391°
−1.945997°
Vinaroz (castellón)
40.461483°
0.507634°
56.026233°
−3.182764°
42.335789° 21.280714° 21.454455° 21.278893° 21.424799°
United Edinburgh Kingdom Boston (Massachusetts) O'ahu, Honouliuli (Hawai) O'ahu, Mokapu (Hawai) O'ahu, Sand Island (Hawai) O'ahu, Waianae (Hawai) USA Orange. San Pedro Shelf (California) San Diego. Point Loma (California) Woods Hole (Massachusetts)
Untreated
0.60
Reference
Estacio et al. (1997)
Del-Pilar-Ruso et al. (2010); De-la-Ossa-Carretero et al. (2012a, 2012b) Del-Pilar-Ruso et al. Near field (2010); Del-Pilar-Ruso et al. (2010); Near field De-la-Ossa-Carretero et al. (2012b) Del-Pilar-Ruso et al. (2010); Near field De-la-Ossa-Carretero et al. (2012b) De-la-Ossa-Carretero Near field et al. (2012a, 2012b) Far field Cardell et al. (1999) Juanes et al. (2004b); No significant Echavarri-Erasun et al. (2007) Del-Pilar-Ruso et al. (2010); No significant De-la-Ossa-Carretero et al. 2012a, 2012b) Far field Franco et al. (2004) De-la-Ossa-Carretero Far field et al. (2012a, 2012b) Far field
1.20
50.00
55,000
1.72
Pretreated 2.49
15.81
7600
0.55
Primary
1.50
30.00
134,587
1.31
No significant Read et al. (1983)
−70.743562° −157.990229° −157.693227° −157.897718° −158.197063°
Secondary Primary Secondary Primary Secondary
16.00 2.00 4.00 2.74 1.50
32.00 61.00 32.00 72.50 32.50
375,000 85,536 51,840 256,608 13,824
0.82 2.30 2.11 2.30 2.32
No significant No significant No significant No significant No significant
33.566966°
−118.004541°
Secondary 8.00
60.00
852,000
1.51
Near field
32.663890°
−117.332071°
41.522697°
−70.673944°
Maciolek et al. (2008) Shuai et al. (2014) Shuai et al. (2014) Shuai et al. (2014) Shuai et al. (2014) Maurer et al. (1998, 2007); Diener et al. (1995)
Primary
7.24
98.00
681,374
1.44
Near field
City of San Diego (2009)
Untreated
0.18
14.00
1175
0.82
Near field
Smith et al. (1973)
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not adequately describe the response of benthos (Maurer, 1993; Echavarri-Erasun et al., 2010). The aim of this paper is to assess the effects of ocean outfalls on adjacent macrobenthic invertebrates, and to test the influence of the outfall characteristics and the physical processes on the magnitude of impacts. 2. Methods Data were obtained from gray and peer-review published literature. Search key words were: sewage, outfall, ocean, benthos (or benthic or macrobenth*). We compiled information from 40 ocean outfalls in 11 countries (Australia, 4; Canada, 2; Chile, 2; Greece, 1; Italy, 4; New Zealand, 2; Portugal, 1; South Africa, 2; Spain, 13; United Kingdom, 1; USA, 8) (Table 1). Each outfall was characterized according to length (m), maximum depth (m), treatment level (untreated, pretreated, primary treatment, secondary or better treatment), flow (m3.day−1) and organic matter mass discharged (kg.day−1). Organic matter was estimated using BOD5 and was calculated from the combination of flow and concentration of effluent considering the treatment level received (untreated: 400 mg/l; pretreated: 300 mg/l; primary: 180 mg/l; secondary or better: 25 mg/l) (WRC (Water Research Center), 1990). Exposure to wave action was used as a proxy for energy of the system where the discharge occurred. Wave action was represented by significant wave height (Hs), mean height (Hsm) and 95-percentile (Hsp95), based on a global wave dataset simulated with the model WaveWatch III and driven by NCEP/NCAR reanalysis of winds and ice fields (Global Ocean Waves, GOW) (Reguero et al., 2012). The magnitude of the impact of the outfall on macrobenthic assemblages was based on the results and conclusions of studies reviewed. The magnitude was categorized according to the detection or not of significant effects and the distance at which significant effects was detected (no effect or very low; significant effect in the near field, b 500 m; significant effect in the far field, N500 m). In addition, the type of ecological effects reported was classified considering the following indicators: i) Effect on community structure parameters (richness, diversity, abundance, biomass, coverage). ii) Changes in the proportion of opportunistic-sensitive species or trophic groups. iii) Changes in the community structure, detected by the multivariate analyses.
To synthesize the types of ocean outfalls and environmental conditions, we applied a combination of two classification techniques, SelfOrganizing Maps (SOM) (Kohonen, 2001) and K-means algorithm
(Hastie et al., 2001). The SOM is a classification method included in artificial neural networks (ANNs) that detects patterns or classes in a set of data, preserving the neighboring relations. This means that similar clusters in the multidimensional space are located together on a 2D grid that allows the data to be intuitively visualized. The starting point of this technique is a data sample in which N is the total number of data points to be classified. The ANN is “trained” using an iterative learning algorithm. The process includes a self-organizing neighborhood mechanism, so neighboring clusters of the winning reference vector in the 2D lattice space are also adapted toward the sample vector, thus projecting the topological neighborhood relationships of the highdimensional data space onto the lattice. In this study, a lattice of 30 neurons (5 × 6) was chosen as an optimum solution. The input variables of the analyses were outfall size and organic matter discharge, as descriptors of outfall characteristics, and depth and wave height (mean, percentile 95), as indicators of the physical environment. The number of groups obtained with the application of the SOM was then clustered by a K-means algorithm. The number of k-mean groups was justified according to the minimum Davies Bouldin Index (DBI) for a solution with low variance within clusters and high variance between clusters (Negnevitsky, 2002). SOM analyses were conducted using Matlab 7.7 and the SOM coding solution based on SOM Toolbox for Matlab 5 (Vesanto et al., 2000).
3. Results Our review of the literature found that for ocean outfalls, most of the studies concluded that there was an absence of any kind of impacts or classified them as not significant (30%) or the effect was restricted to the plume mixing zone or the near field within 500 m of the outfall (50%). The main types of effects reported were changes in the proportion of opportunistic/sensitive species (89%), a decrease in richness and/or diversity (61%), an increase in total abundance (54%), and changes in community structure (39%) (Fig. 1). Almost half of the ocean outfalls included in the study discharged wastewater without previous treatment or only pretreated, and most of them (82%) were reported to have a significant effect, both near field (47%) and far field (35%) (Fig. 2). The three levels of impacts considered were detected in treated discharges, although none of the outfalls with secondary treatment showed an impact beyond the near field. With respect to flow and mass loading it stands out that all of the biggest outfalls were reported to have a significant effect, although some of those with very high organic matter input did not seem to have any kind of impacts. On the other hand, even some of the smallest outfalls showed a significant impact in the far field. Impacts were detected in a wide range of depths, although a high proportion of
Fig. 1. Type of effects reported in the papers reviewed (number of outfalls).
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those discharging in deep bottoms had a significant impact, both near and far field. Finally, it should be highlighted that none of the outfalls located in very exposed environments showed a deleterious effect in the far field. Patterns emerged when gradients in both organic matter inputs from the outfalls and the energy of the environment were plotted together (Fig. 3). No significant effects were observed when the outfalls fulfill the following characteristics i) low organic matter load (b 10,000 kg.day−1), even in sheltered environments (Hsm b 1 m), ii) medium to very high organic matter load (N10,000 kg.day−1) located in exposed or very exposed sites (HsmN 2 m). Outfalls with reported significant effects even beyond the mixing zone exhibit the following conditions i) low organic matter load, located in sheltered environments; ii) high organic matter load (N 20,000–30,000 kg.day−1) located in semiexposed
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environments (Hsm N 1–1.5 m) and, iii) very high organic matter load (N 30,000 kg.day−1), except in very exposed environments (Hsm N 2 m). A mixture of secenarios appeared when the deleterious effect was restricted to the near field, although most of the cases were outfalls with low–moderate organic matter located in sheltered environments, or with high–very high matter load discharging in semiexposed sites. From the SOM and K-means analyses, the minimum Davies Bouldin index obtained determined that 4 was the optimal number of groups for the K-means analysis. The range of variation of each variable was represented on the trained SOM. In addition, with the aim of facilitating the interpretation of the results we have represented the magnitude of the effect considering a gradient from 1 to 3 (1 = no significant effect, 2 = near field effect, 3 = far field effect). Based on these results it was possible to observe how the outfalls were grouped according to their
Fig. 2. Number of outfalls reported as not having deleterious effects and those with demonstrated significant effect (near and far field) in different categories of the factors analyzed.
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intrinsic characteristic and the physical energy in the surrounding environment (Fig. 4). The main characteristics of the four groups identified by K-means analysis are: - Group A. It comprises discharges with low to moderate organic matter input (from 12,000 to 75,000 m3.day−1; 346 to 22,000 kg.day−1) and all of them located in very exposed environments except Hastings (Hsm = 1.59 m) and San Sebastián-Pasajes (Hsm = 1.72 m). The level of treatment ranges from untreated to secondary. Most of the outfalls in this group did not show a significant effect (Black Rock, O'ahu-Waianae, O'ahu- Mokapu, Penco, Santander, Tomé), although two of them were reported to have an effect in the near field (Gisborne and Hastings) and one even in the far field (San Sebastián-Pasajes). - Group B. It includes outfalls with medium to high organic matter discharge (from 437,754 to 567,000 m3.day−1; 15,396 to 102,060 kg.day-1). All of them were untreated or with primary treatment. The environment was exposed (Hsm N 1.7 m) or very exposed (those of Hawai Hsm = 2.3), and only the Lisbon outfall was in a somewhat sheltered environment (Hsm = 1.45 m). In general, these group clusters discharges with moderate impacts in the near field (Bondi, Durban (Central), Durban (Southern), Lisbon, Mcaulay Point, North Head), although two of them were considered to have a very low impact (O'ahu-Honouliuli, O'ahu-Sand Island) and one a significant impact even in the far field (Strait of Georgia). - Group C. This group includes outfalls with low to moderate organic matter input (from 1175 to 375,000 m3.day−1; 36 to 24,226 kg.day−1) located all of them in sheltered environments (Hsm b 0.8) or semiexposed environments (Edinburgh, Hsm = 1.31). The magnitude of their impacts was in most of the cases moderate (Algeciras, Barcola, Canet, Gandía, Grado, Oliva, Peñíscola, Porto Busto, Trieste/ Servola, Woods Hole), but some of them were considered not to have an impact (Boston, Edinburgh, Lignano, Torreblanca); and three had a deleterious effect even beyond the mixing zone (Alcossebre, Benicarló, Vinaroz). - Group D. It contains outfalls with high organic matter input (from 400,000 to 852,000 m3.day−1; 21,300 to 240,000 kg.day−1) and located in semiexposed environments (Hsm N 1.5), except Saronikos Gulf
where Hsm = 0.35 m. This cluster includes the outfalls with the highest impacts (Sant Adriá del Besós, Saronikos Gulf), although in those of Malabar, Orange and San Diego the impact seems to be restricted to the mixing zone.
4. Discussion Our main conclusion is that impacts linked to ocean outfall discharges are generally lower than expected relative to impacts from embayment outfalls. The evolution of sewage treatment and relocation of outfalls from within Boston Harbor, USA, to an ocean outfall provide a good example (Signell et al., 2000; Diaz et al., 2008). Most significant ocean outfall effects detected are limited to the near field (b 500 m from the outfall). These results support the statement of Roberts et al. (2010) that properly designed ocean outfalls do not cause significant ecological impacts. The widely applied model described by Pearson and Rosenberg (1978) for response of benthos to organic matter gradients needs to be revised for coastal areas. The balance between the total organic mass loading to bottom waters and the energy of the system seem to be the key factors in determining benthic impacts. Thus, when the amount of organic matter is low, the environment is usually able to assimilate the contamination, even in sheltered environments and untreated effluents. Moreover, even big outfalls do not show a measurable deleterious effect beyond the near field, when they are located in exposed environments or appropriately treated. The combination of a range of organic matter inputs with a gradient in the exposure to waves, from sheltered to very exposed environments, results in a wide range of magnitude and spatial extent of impacts. The availability of oxygen in the sediments is involved in the variety of responses of benthic communities in the studies reviewed. The depletion of oxygen by decomposition of organic matter is perhaps the most important factor in the structuring of benthic communities and the ecosystem functioning in polluted sites (Pearson and Rosenberg, 1978; Diaz and Rosenberg, 1995, 2008; Conley et al., 2009). However, in the open sea the high initial dilution (at least 100:1), the intense mixing and the large surface area available for re-aeration makes it difficult to
Fig. 3. Number of outfalls reported as not having deleterious effects and those with demonstrated significant effect (near and far field) in different scenarios combining the rate of organic matter loading (Low: b10,000 kg.day−1; Medium: N10,000–20,000 kg.day−1; High: N20,000–30,000 kg.day−1; Very high: N30 000 kg.day−1) and the mean significant wave height (Sheltered: b1 m; Semiexposed: N1–1.5 m; Exposed: N1.5–2 m; Very exposed: N2 m).
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generate hypoxia, especially in those environments with high wave height or currents (Roberts et al., 2010; Conley et al., 2009). In addition, despite of the ecological importance of hypoxia most marine invertebrates are not significantly affected until very low concentration (Diaz and Rosenberg, 1995). Several authors have established the thresholds for sublethal effects between 2.0 and 2.8 mg O2 l−1 (Tyson
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and Pearson, 1991; Diaz and Rosenberg, 1995; Vaquer-Sunyer and Duarte, 2008), a concentration that is typically reached under very high organic input and poor flushing conditions (estuaries, enclosed seas, stratified water bodies). The relationship between water renewal and organic enrichment has already been reported before for coastal areas or even estuaries.
Fig. 4. A) Gradient analysis of each physical variable on the trained SOM, with visualization in a shading scale (light = high values, dark = low values). B) K-mean results on the SOM plane, (A, B, C, D denotes the four groups identified). C) Visualization of the average magnitude of the impact in each neuron, with visualization shading (blue = low impact, red = high impact). The outfalls included in each neuron are indicated.
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For example, Roper et al. (1989) found little effect in the macroinvertebrate assemblages near a sewage outfall discharging 69,000 m3.d−1 into an estuary, in Tauranga Harbour, and concluded that is probably related to the high water velocities over the outfall diffuser (up to 0.5 m s−1). Pearson and Rosenberg (1978) in describing macrobenthic succession along a gradient of organic enrichment, held that oxygen would not be a limiting factor in well-flushed bottoms and an increase in benthic biomass could be expected, stimulated by excess organic matter in the ecosystem. Therefore, based on these premises and supported by the SOM trained map from our analysis we formulated a “model” in which the lower the organic matter and the higher the energy of the system, the lower the impact. However, we found some exceptions to this general statement in the papers reviewed. For example, lower impacts would be expected in the case of Gisborne, Hastings and San Sebastián-Pasajes outfalls (Group A), because all of them are quite small and located in exposed environments. Nonetheless, regarding the two New Zealand outfalls, Roper et al. (1989) remarked that no grossly polluted zone devoid of macrofauna was found and the polluted zone was limited to within 200 m of the diffusers. In addition, it should be taking into account that both Gisborne and San Sebastián are untreated, and the later discharge is close to a previously degraded area. The outfalls of O'ahu-San Island (Group B) were expected to have a higher impact considering their size, but the energy of the system likely minimized impacts. As well, Lignano and Torreblanca (Group C) did not show a significant impact despite the extremely sheltered conditions at their discharge points, which is related to their small size and secondary treatment. Finally, the outfalls of Orange, San Diego and Malabar (Group D) seem to have a less impact than expected according to the model for their size. Orange and San Diego are very long (N7 km) and deep (70 and 98, respectively) which increases dilution capacity. Malabar is shorter but the discharge point is also quite deep (60 m). Similarly, the outfall of Boston (Group D) did not show any deleterious effect, despite discharging a high organic matter load in a sheltered environment. This outfall has secondary treatment, is extremely long (16 km with 2 km of diffusers) and quite deep (32 m). These results emphasize the importance of the treatment and the outfall design in minimizing environmental impacts. In other cases, the unexpected results could be explained by an over or under estimation of the system's energy. For example, the wave height in the Strait of Georgia (Group B) may be lower than that estimated due its geomorphology, and the currents in Edinburgh (Group C) may contribute to increase the water renewal. Most of the significant impacts in the papers reviewed frequently described a decrease of richness and diversity, an increase in total abundance and changes in the composition of community, including a decline of abundances of sensitive species and increase of opportunistic species. Pearson and Rosenberg (1978) conclude that in polluted sites large individuals and long-lived equilibrium species are eliminated and population shifted toward younger individuals, and smaller and more short-lived species that possess opportunistic life histories. However, none of ocean outfalls evaluated reported a complete absence of fauna or a total dominance of opportunistic species as would correspond to the first stage of succession in a process of organic enrichment. Moreover, some of the assemblages colonizing the bottoms around the ocean discharges correspond to the ecotone point, with low richness and high abundance, or the transition zone, characterized by great fluctuations of the populations, described as successional stages between the peak of opportunistic species and the mature and stable community in the Pearson– Rosenberg paradigm. In summary, we can say that the general pattern of succession of benthic communities brought about by ocean outfalls fits the model of Pearson–Rosenberg with some deviations i) the probability of a significant impact is much lower ii) not all the successional stages occur and, iii) the magnitude of the changes are usually lower.
Acknowledgments The authors are very grateful to José A. Juanes (IH Cantabria) and Carlton Hunt (Battelle Memorial Institute) for organizing the International Workshop on Marine Outfalls Discharges (IWMOD 2010), which was the origin of this paper. We also thank to Elvira Ramos for her help in the statistical analysis.
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