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