Cefas response to Bruce Selkirks
Cefas response to Bruce Selkirks note “Whitesand bay and the Cefas report - some areas of serious concern” (prepared by Bruce Selkirk, July 2005)
We have presented our comments under the same headings as they appear in the note.
Introduction
We wish to acknowledge the significant effort expended by Dr Selkirk on this critique of the Cefas Rame Head environmental assessment. We hope that our response will be helpful in clarifying some of the issues raised and, where appropriate, in identifying Cefas’ intentions for follow-up work.
1. Methods used
We should clarify that the methods used for the analysis of organic contaminants at the disposal site were the same as those used to analyse sediments from the dredged areas and at all other locations discussed in the report. Comparison of contaminants with respect to Polycyclic Aromatic Hydrocarbons (PAH) was made between the disposal site, the point of dredging and the surrounding area. This was done by apportionment of the PAH to oil or combustion categories. The results showed that the PAH signature in this area was predominately combustion-derived, which might suggest a more generic (i.e., urban as well as industrial) source of contamination than those related to shipping activities alone. Cefas is in the process of conducting further work to investigate the sources and extent of PAH contamination in the area, which will include the determination of “signatures” for some organic contaminants.
2. Errors and paucity of data
As at other locations, we accept that the scope for making comparisons with earlier environmental studies off Plymouth can be limited by differences in objectives and methodology. However, we also feel that the alternative, namely to make no reference to historical information, would be unacceptable. As part of ongoing work, Cefas are committed to further exploration of possible data sources in collaboration with local laboratories (and perhaps Dr Selkirk) in the hope that at least some may be helpful in our future assessments.
We do not consider that the report makes a strong claim that sewage outfalls are to blame for any observation. For example, regarding PAH, the report states clearly that sewage outfalls and storm drains are potential mechanisms for delivery of contaminants into the system, and should be investigated further. Updated information on these activities may provide further insight into contaminant sources within and adjacent to the Whitsand Bay area.
We are uncomfortable with the designation of the marine environment in the near vicinity of Plymouth as a rural location, just as it would be inappropriate for the waters adjacent to the Tyne Mouth. The localities are subject to varying degrees of natural and anthropogenic influences arising inter alia from estuarine efflux and a long association with maritime affairs, and both merit consideration in the interpretation of survey outcomes. For example, the sediments of the Tamar Valley and its catchments contain naturally high concentrations of many metals. The Cefas report also offers a wider (national) perspective on contaminant levels in order to assist in the interpretation of local findings.
3. Conflicts between data obtained and conclusions drawn
3.i. Effects on biodiversity
The observation on the lower range of species present in the environs of Polhawn Cove (which we assume relates to station G3) can be explained by the nature of the sediment in this area. This station is located in shallower water than other sampled locations, and hence the seabed will be naturally more exposed to physical disturbance from wave action, in addition to tidal influences. As a reflection of this, the sediment at this location is a well-sorted clean sand and as such will support species which are ecologically suited to this habitat. Such habitats typically support a lower range of species than a habitat which is located in deeper water (and hence less likely to encounter physical disturbance from wave action) and which exhibits a more mixed sedimentary environment which can be exploited by a greater number. Our data therefore conform with expectation regarding the influence of natural forces. It should also be noted that correlation analyses of the biological communities present and levels of contaminants showed no significant relationships.
Dr Selkirk deduces a causal link with transported dredged material from the higher mean values for trace metal concentrations ‘adjacent to Polhawn Cove’ relative to the disposal site. We consider that it might be more plausible, though equally speculative, to conclude from these values that the regularly-used disposal site was an unlikely source of inshore elevations, since levels here (where, supposedly, all the problems emanate from) were lower. On the sub-set of stations used for determining average values, RH4 and RH5 are indeed ‘adjacent to Polhawn Cove’ but surely not G13, which is near to the former disposal site. Here, trace metal concentrations were appreciably higher in 2003 than in preceding or subsequent years. Planned work by Cefas will provide further insight into contaminant status at this location (as well as others) and will help to determine how representative the 2003 results were. Given its proximity to the historical boundary of the site, we would not dismiss the possibility that the patchy local occurrence of elevated contaminants there might represent a legacy of past disposal.
3.ii. Effects on viability
Cefas employed standard test procedures for the two bioassays, along with conventional statistical tests which revealed no significant between-station differences in mean % survival of either test organism. We therefore have no defensible evidence of an effect but this does not preclude conjecture on the meaning of any nonsignificant patterns. This is explored in Dr Selkirk’s note, although it is partial in its treatment of the data. Thus reference to the occurrence of highest mean % mortality of Corophium at station G2 to the west of the disposal site, which was characterised by relatively low contaminant concentrations, is omitted. At both G2 and G13, the Cefas report noted that the sediments had a coarse component, comprising shell and gravel. We should clarify that the experimental evidence suggested that this was the main factor in causing a higher and - in the case of G2 - a more variable mortality rate between replicates, by restricting the ability of these amphipods to burrow into the substratum, causing stress that was not toxicant-related.
Cefas plan to conduct further confirmatory bioassays at the same stations in 2006.
3.iii. Bacterial mats
The prospects for the inshore accumulation of particulates in sufficient quantity to eliminate macroalgae are addressed under 4., below. Employing evidence on the occurrence of Beggiatoa elsewhere, we consider that it was reasonable to explore the possibility of a local source of organically-rich waste input, including any local sewage discharges to the area. A national survey by Cefas identified that dredged material from certain urbanised estuaries may contain significantly elevated levels of organic carbon, but this was not the case for the Plymouth/Tamar area. Also, the implausibility of ascribing Beggiatoa growth inshore to organically-enriched dredged material is accentuated by its absence at the regularly-used disposal site.
Cefas staff plan to consult with local interests, including divers and the MoD, in further exploring the occurrence and possible causes of Beggiatoa growth in the area.
4. Tidal flushing
The report carefully considered the various processes which operate in the different parts of the broad extent of Whitsand Bay. For the disposal site itself, there is a wide range of direct and indirect evidence for some accumulation of dredged material, but relatively little accumulation in the long-term. Regarding the shallow sub-tidal zone landwards of the disposal site, the grain size and chemical data provide no evidence to indicate a build-up from this source.
The report describes in detail the capacity of the physical dynamic system of Whitsand Bay to transport fine sediments to all areas. It also outlines the potential for accumulation in Polhawn Cove, i.e., the intertidal and immediate shallow subtidal areas.
The variables of the plume model are fully described and the model results are presented. Whether or not the model describes “somewhat ideal dispersal conditions” depends on what is considered ideal, between end members of complete dispersal and complete non-dispersal. For clarity, we emphasise that this is a ‘plume model’, i.e., it refers to the potential movement of individual particles settling through the water column. Thus it is useful as a simplified indicator of potential movement of a small proportion of the disposed material and, for example, provides a first-pass indication of whether dispersed material from individual disposal events has the potential to travel landwards rapidly and impact the Scylla and other dive sites.
The plume model does not attempt to describe the later movement of the bulk of material which reaches the bed - for this, we have direct evidence (swathebathymetry, side-scan sonar, grab samples), as well as a conservative accumulation budget derived from such observations. Nor does it describe the movement of any near-bed turbid plume generated during disposal (data at the Scylla exist but are not ideal for assessing this possible impact).
The weight of evidence points to no long-term accumulation of dredged material in the area. The report does not state or infer that “all spoil is transported out from the Bay and Cove”. On the contrary, it describes the accumulation of some material at the disposal site. The extent of knowledge of the potential mechanism(s) for transport of material into the area, including Polhawn Cove, is also described in the report. Further, it provides no evidence to suggest that there is a build-up of organic contaminants or trace metals in sediments sampled near Polhawn Cove.
Finally, we emphasise that our plans for future monitoring and information-gathering in this area will encompass further consideration of transport processes for validatory purposes.
An alternative scenario?
Dr Selkirk provides an interesting and complex scenario, where the nature of the supporting evidence is very limited, and one which he readily admits is conjectural. For monitoring outcomes at Rame Head, we are inclined towards a consideration of simpler explanations for the wide-ranging and internally consistent suite of observations presented in the Cefas report. However, the scenario is clearly not specific to Rame Head and presents a legitimate discussion point among scientists engaged in the general issue of particulate transport and fate at sea. This is outside the scope of a routine monitoring programme to address, but Cefas scientists would be happy to consult further on the matter.
On the final comment under this heading, we do not consider that the arguments or evidence presented justify changes to the key conclusions of the Cefas report.
Cefas
28th September 2005
