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IS THERE REASONABLE EVIDENCE TO SUGGEST THAT THE NEW MASSACHUSETTS WATER RESOURCES AUTHORITY SEWAGE OUTFALL MAY HAVE A NEGATIVE IMPACT ON LOBSTERS?
THE LOBSTER CONSERVANCY
P.O. Box 193
Prepared by: Kari L. Lavalli, Ph.D, Vice-President in consultation with Diane F. Cowan, Ph.D, Executive Director April 22, 1998
The following concerns have been raised by Dr. Joseph Ayers (of Northeastern University's Marine Science Center) in a memo to Jerry Schubel, Chair of the MWRA Outfall Monitoring Task Force (dated 9 June, 1997) and a letter to John DeVillars, Regional Administrator of EPA (dated 23 December 1997):
Dr. Ayers has proposed that the potential negative impacts of the new MWRA outfall be studied on early life history stages of lobster, namely, the larvae, postlarvae, and young-of-the-year (YOY). Based on a critical review of the literature, we will argue that many of the concerns raised by Dr. Ayers can be eliminated due to a large body of existing data. This report will: (1) summarize the results of studies that directly relate to the above-mentioned concerns, (2) point out where knowledge is lacking, and (3) present suggestions for further research. We will begin by attempting to establish the proper context for evaluating the concerns by (1) briefly summarizing what is known about early lobster life history, behavior, and ecology; and (2) discussing the validity of indicator species being used by MWRA. Then we will review the toxicity literature and discuss whether results have indicated a cause for concern at the site. Finally, we will offer some concluding remarks. This report is not meant to be a replacement for the ENSR Consulting and Engineering Report of 1 March 1998 (prepared by Mitchell et al. for the MWRA), entitled "Massachusetts Bay Outfall Monitoring Program: Toxics Issue Report on Biology of the Lobster in Massachusetts Bay" -- instead it is meant as a supplement to that report.
I. LOBSTER BEHAVIOR: What is the likelihood that lobsters will come into contact with the new MWRA outfall in Boston Harbor?
Larvae:(Note on terminology: Here we will refer to larvae by convention as pelagic stages I-III) Upon hatching, lobsters molt out of the pre-larval stage into the first larval stage. These animals are attracted to light and make their way to the upper couple of meters of the water column where they are transported predominantly inshore by wind-driven currents. Studies examining the position of larvae in the water column indicate strong differences depending on whether the larvae are found offshore (in deep ocean waters), or inshore (in shallower waters). Since Boston Harbor is inshore and the outfall pipe is in "relatively" shallow water (~100 ft or 28-32 meters), this discussion focuses on larval behavior in shallow waters.
In coastal waters, larvae (stages II-III) and postlarvae are confined to the upper 2-3 m of the water column; however, stage I larvae exhibit semidiurnal vertical migrations that are based on light intensity -- these movements concentrate them at the surface in early morning and late afternoon/early evening and involve depth changes of several meters (Harding et al., 1987; Scarratt, 1973; Templeman and Tibbo, 1945). Some of these vertical movements may allow the larvae to utilize subsurface countercurrents to counteract surface currents to remain near a certain location -- such subcurrents are used by other decapod larvae. Vertical temperature and salinity profiles, as well as the depth of the thermocline, strongly influence the distribution of the larvae. Larvae are repelled by freshwater so, after a heavy rain, they will be repelled by the freshwater layer on top of the seawater -- similarly, they would be repelled by freshwater runoffs in nearshore environments and the MWRA effluent if they were even found that deep. Larvae actively avoid swimming toward seawater diluted to 21 ppt, but will readily swim into salinities ranging from 26.7 ppt to 32 ppt (Scarratt and Raine, 1967). They are not terribly strong swimmers and cannot fight against currents -- which would be present at the site of the diffusers given the effluent discharge rate out of individual diffuser ports of ~5.6-22.6 ft/sec or ~170-690 cm/sec (Ken Keay, MWRA, personal communication). Thus, their exposure to low salinities would be questionable, first due to the depth of the diffusers and the effluent and second due to their inability to swim towards the diffusers during emissions of effluents.
Thermoclines, particularly strong thermoclines (>5°C difference in upper vs. lower waters), restrict both larvae and postlarvae to the warmer upper layers. At the location of the outfall diffusers there is a strong thermocline at about a 5 m depth (MWRA, 1997) with a 5-10°C temperature stratification between surface and near-bottom waters from roughly May to Late October (Ken Keay, MWRA, personal communication) -- months which represent the time when larvae would be present in the water column. The thermocline present indicates that the larvae, if present in the waters above the discharge, will be restricted to the upper waters away from effluent discharge. Furthermore, as the effluent is freshwater and will cause a lowering of salinity in the immediate vicinity, larvae if present at all at depth, will be repelled from the area.
The two main points concerning larvae are that (1) the depth of the outfall pipe (28-32 meters) does not coincide with the vertical depth to which larvae are typically found (1-3 meters), and (2) the presence of a sharp thermocline at 5 m depth further prohibits their likelihood of movement into the range of an effluent plume. Furthermore, given that the effluent's temperature will range from ~13°C to ~24°C at the location of the plant, and will cool over the 9.5 mile pipe (as heat is given up to the surrounding cooler bedrock through which the pipe travels), its temperature upon mixing will be reduced to a level that will prevent it from rising past the thermocline. Current estimates for the temperature of the mixed effluent, based on a turbulent mixing rate of 49:1, indicate that it will be between 8.3 to 12.25°C (Ken Keay, MWRA, personal communication). As the effluent plume rises, it will continue cooling and by the time it reaches the thermocline 15 meters above the benthos, it will not be any warmer than the ambient seawater. Thus, the thermocline will prevent the effluent plume from reaching the upper water layers and it is unlikely that larval lobsters would ever come into direct contact with substances from the wastewater outfall.
Postlarvae:(Note on terminology: Here we will refer to postlarvae as the transitional stage between the pelagic and benthic realms--this is the settling stage and is not a larval stage. Larvae do not settle or come into contact with the benthos at any time). After 3 larval stages, lobsters molt into the postlarval stage (stage IV). At this stage, they look like miniature adults (except their claws are small) and they are excellent swimmers. They begin to make excursions to the bottom to search for suitable habitat. During the transition from pelagic larva to benthic postlarva, the lobster is sensitive to temperature, current velocity, salinity and depth.
Postlarvae are strongly influenced by the presence of thermoclines -- a difference of 5°C is all that is necessarily to significantly reduce the possibility that the postlarvae will continue their downward journey to the bottom (Boudreau et al., 1992). At the site of the outfall, there is a strong thermocline present at a 5 m depth representing 5-10°C of temperature stratification between surface and near-bottom waters from roughly May to late October (MWRA, 1997; Ken Keay, MWRA, personal communication). Thus, few, if any, postlarvae are expected to pass through this thermocline to examine the bottom habitat near the outfall. Furthermore, smaller juvenile lobsters (>16 mm CL) are sensitive to currents and tidal action and can walk normally in current speeds of 5 cm/sec, but are impaired at currents of 10-15 cm/sec (data on lobsters having difficulties in flow regimes is from a study on a sister species of lobster by Howard and Nunny, 1983). As the effluent discharge rate at the diffuser nozzles will be on the order of 170-690 cm/sec, currents well in excess of 5 cm/sec will be generated, making it not only impossible for the postlarvae to maintain their position, but also making it impossible for YOY to accomplish any kind of movement on the substrate surface. Thus they will not be found in this area.
Finally, all sampling data thus far accumulated indicates that postlarvae settle in shallow coastal bays -- from the lower intertidal/shallow subtidal interface (Diane Cowan and Jay Krouse, unpublished data) to between 5-10 m in depth in the subtidal (Wahle and Steneck, 1991). Samples taken at 20 m depths show an order of magnitude drop off in the numbers of YOY lobsters present with these numbers being almost undetectable (Robert Steneck and Carl Wilson, unpublished data).
While postlarvae are capable of using a variety of habitats (ranging from cohesive mud to peat beds in salt marshes to eelgrass to boulders on shell hash -- sometimes termed "cobble"), environments comprised of rocks and boulders represent the "safest" habitat against predation events. Featureless habitats (those lacking in structure or vegetation) represent the "worst" habitats. This does not necessarily mean, however, that lobsters only settle in "cobble" habitats. The habitat near the outfall diffuser pipes ranges from a sand-gravel bottom between submerged, relict glacial drumlins to a cobble habitat with some boulders some distance away. This habitat, while certainly suitable for postlarval settlement, would most likely not even be sampled by the postlarvae due to the thermocline present and the depth involved (>20 m). Furthermore, freshwater plumes would repel the postlarvae and flows out of the diffuser would prevent postlarvae from settling nearby. Additionally, no evidence to date indicates that this nearby cobble area would represent a "prime habitat for larval [sic] recruitment" prior to diffuser deployment (as stated by J. Ayers in his memo to Jerry Schubel, Chair of the Outfall Monitoring Task Force, dated 9 June 1997).
Wahle and Steneck (1991) have suggested that availability of shelter-providing habitats may limit postlarval recruitment to the benthos. This is not to say, however, that all shelter-providing habitats represent recruitment habitats. Evidence for resource- or recruitment-limitation has been conflicting. A previous article by Incze and Wahle (1991) suggested that habitat quality shaped recruitment patterns irrespective of postlarval supply. Their most recent study (Wahle and Incze, 1997), representing many more years of study, now favors the hypothesis that it is postlarval supply that drives benthic recruitment. Even where special "cobble emplacement plots" were placed so as to promote postlarval benthic recruitment, Wahle and Incze did not find settlement in areas where postlarval supply was low. Thus, the cobble habitat located near to the outfall pipe would only be a potential site for benthic recruitment if postlarvae of molt stage Do or greater (these are the "competent-to-settle" postlarvae) were present in the water column above, if they were inclined to pass through the thermocline present, and if they were inclined to swim for more than 20 m to reach the bottom below. Their presence or absence can easily be assessed via neuston nets towed outside a vessel's bow wake during settlement season from July through October (as per Wahle and Incze, 1997).
The take-home message concerning postlarvae is that they are unlikely to ever come into proximity to the new MWRA outfall.
Adults: (Note on terminology: Adults are defined as animals who are both physiologically and functionally mature. Ovigerous females are called "eggers" by lobstermen, while recently molted lobsters are called "shedders".) Of all the life history stages, free-ranging adult lobsters are the most likely to come into contact with the new outfall pipe because they are known to make seasonal movements and are capable of traveling great distances. The most relevant questions, therefore, concern the adult populations -- their distribution and range of movements in the outfall area and the effect that effluent compounds may have after long-term exposure. To date, we have very limited data in most areas of the kinds of movements adults make and we do not know whether or not most coastal lobsters are residential or transient. Collecting such data would require tag and recapture studies. The most logical way to capture the lobsters is in commercial traps; however, the required coordinated cooperation between scientists and fishermen may be prohibitive because the study would be labor intensive for the fishermen and would take time away from fishing. Additionally, lobstermen have been loathe to provide location data since that would tell others where they are laying their traps; however, scientists could alleviate such fears by showing the lobstermen the kinds of movement maps that would be constructed from their location data (and these would not show points, only lines -- from which it would be difficult, if not impossible, to determine exact locations). So while previous such studies have only had limited success, future studies could have greater success if the scientists and lobstermen took the time to discuss what would happen to the data collected and discussed results after they were collected and analyzed. Such studies have been successful in Canada and in Cape Cod waters (Estrella and Morrissey, 1997). Interestingly, Estrella and Morrissey found that ovigerous females moved significantly more than sublegal and legal-sized females without eggs. These movements will typically be offshore for females who have just extruded and inshore for females preparing to hatch their eggs and molt (these movements maximize temperatures needed for egg development). This movement pattern suggests that developing embryos would have limited exposure to the effluent, if ovigerous females were located nearby. Furthermore, larger lobsters are also affected by current speed: Adolescent lobsters (50 mm CL) can walk normally in current speeds of 5 cm/sec, but their walking is impaired at 10-15 cm/sec and at 21-42 cm/sec, they can no longer gain purchase on the substrate and slip downstream (Howard and Nunny, 1983). As mentioned above, the effluent discharge rate at the diffuser nozzles will be on the order of 170-690 cm/sec, which would discourage larger lobsters from venturing nearby.
Conclusion: Given that the larvae are restricted to the warmer upper water layers and exhibit behaviors that prevent them from staying in the presence of lower salinities and colder temperatures, it is highly unlikely that they will come into contact with the outfall discharge. Similarly, existing evidence indicates that postlarvae avoid passing through thermoclines with a temperature difference greater than 5°C and, while their tolerance to low salinities is higher than that of the larvae, postlarvae prefer salinities above 26 ppt. Finally, all evidence to date indicates that regardless of whether a habitat is suitable for settlement, if postlarvae are not present at molt stage D0 in the overlying waters, benthic recruitment will not occur.
Dr. Ayers proposed benthic sampling via SCUBA and airlift techniques to determine whether or not benthic recruitment to the cobble habitat adjacent to the outfall occurs. Given the depths involved (~30 m), this kind of sampling would require the repeated use of NITROX diving over a long period of time to sample the necessary number of quadrats for an adequate sample (~four 0.5 m2 quadrats could be sampled in the 40-50 min allowed for NITROX 36 mixtures at 27-30 m). This kind of sampling is not necessary given that the probability of benthic recruitment can be assessed via neustonic sampling for postlarvae of molt stage D0 in the overlying waters from July through October. If no such postlarvae are found, there will be no benthic recruitment to this cobble habitat. Furthermore, neustonic sampling is more cost effective as it requires only boat time for the tows, while benthic sampling requires boat time, SCUBA costs, extra personnel costs -- divers, and also exposes divers to potential hazards. However, if benthic sampling is conducted, it should be understood that the absence of lobsters in the size range of 5-25 mm CL would mean that no recruitment is occurring at the cobble site adjacent to the diffusers. Lobsters of a size 25-40 mm CL are known to immigrate from their settlement sites to other sites (Wahle and Incze, 1997), and if present at all should not be considered to have necessarily settled at this cobble site.
Dr. Ayers has also proposed chronic toxicity testing of caged early benthic lobsters at the cobble habitat adjacent to the outfall diffusers, control "clean" regions, and the present Deer Island outfall. He has proposed to do this using ten 1 m2 wire trays enclosed with screening, and filled with cobble placed on the bottom. After several weeks of "seasoning", he has proposed to add 10 YOY lobsters and retrieve them 2 months later. This study is impractical for the following reasons:
Even if all of the problems describe above were solved, this study is not necessary until we know whether or not competent postlarvae are present in the waters overlying the outfall area. The only studies warranted at this time are those determining the presence of competent postlarvae in water column above the outfall site and possibly those involving the movements of ovigerous females described below (under section III. Toxicity Issues).