Moville and Green Castle Proposed Sewage Treatment Plant
Considering Impacts on Sea grass Zostera marina
Karin Dubsky, BA, MSc Coastal Ecologist
Author:
Lecturer and scientific research staff, civil and env. eng. Trinity College Dublin and head of
the environmental group Coastwatch. Primary degree in Zoology, Masters in Environmental
Sciences with special focus on estuarine biology. Decades of coastal research, management
and site restoration experience in Ireland and abroad.
Report
This short report sets out the value of and likely damage to the sea grass beds in the area
where the proposed Moville and Green Castle sewage treatment plant outfall is to be sited.
It concludes with recommendations.
Content:
1. Introduction
1.1 Zostera marina
Sea grass as species and habitat
Legal Status
1.2 Coastwatch Survey work and results
1.3 Goals of Sewage Treatment.
2. Sewage discharge pipe construction phase
2.1 Direct impact of digging the pipe trench
2.2 pipe trench spoil
2.3 Sediment plume from construct
3 Recovery from construction phase
4 Sewage treatment normal operation phase:
4.1 Warm treated fresh water plume fate on Zostera bed seaward edge
4.2 Nutrient enrichment and residues
5 Sewage treatment emergency overflows:
6.1 Raw discharge with sanitary material debris caught in Zostera
6.2 Oyster - L Ryan case
6. Conclusions
7. Recommendations
1. Introduction
1.1 Zostera marina Sea grass as species and habitat
Zostera marina is a marine flowering grass, which forms dense beds, with trailing leaves
swimming in the water column. The grass can be up to 1m long (up to 2 m in
exceptional circumstances (Brittany France) (Hily et.al. 2003). Beds can be very long
lived > 100 years.
To develop and maintain itself the Zostera bed require shelter, the right sandy or
muddy sand stable substrate, clean transparent water, yet enough nutrients. So we find
it in sheltered bays and lagoons from the lower shore to about 5 m occasionally down to
10m depth.
Ecological significance: Seagrass stabilises the substratum as well as providing
shelter, a substrate and foraging area for many organisms.
Organisms may be grazing on the blades or sheltering in grass or roots. Or hidden in the
micro habitats around the rhizomes, roots and in the sediment. As recent example of
species richness in Irish sea grass the National Parks and Wildlife Service Manual
reports a Cork Kerry study of Zostera marina by UCC scientists, where a total of 124
species representing 81 families from six phyla were recorded from infaunal
samples.Thirty-two of these species were only found at Barloge Creek, while 45 were
only found at Ventry Bay and a further 47 were found at both locations.
( Irish Wildlife Manual No 28: Dale, A. L., McAllen, R. & Whelan, P. (2007)
Management considerations for subtidal Zostera marina beds in Ireland. [2,008KB)
Distribution and Trends
Zostera marina beds, bed sizes and health have declined over the last 80 years across
the world and in Europe (see OSPAR Review) In Ireland it is officially known to occur in
19 bays. However Coastwatch volunteers have found further beds in at least 3 bays
including the beds in Lough Foyle.
The head marine biologist for NPWS, Dr. Eamon Kelly supplied the following most
recent brief status information on Z marina stocks to the OSPAR Convention as
published on their website: ‘The habitat is under threat in Ireland (Kelly, pers.com 07).
Halting and reversing that trend
In line with Biodiversity Convention goals governments are looking at ways to protect,
manage and restore this species – important in its own right and because of the many
ecosystem services it provides to other species including man. The OSPAR Commission
Zostera report notes: ‘While reductions may be rapid, re-colonisation may require long
time periods. Once seagrass habitats are lost, restoration (if possible) is therefore likely to
require considerable resources. Efficient management must therefore focus upon
maintaining existing populations through protection of habitats (Krause-Jensen et al.,
2004). Krause-Jensen, D, ED Almela, AH. Cunha and TM. Greve. 2004. Have seagrass
distribution and abundance changed? In Borum J, CM Duarte, D Krause-Jensen and TM
Greve eds. 2004. European Seagrasses: An introduction to monitoring and management.
The M&MS project. ISBN: 87-89143-21-3. Internet version: http://www.seagrasses.org
Legal Status
Zostera marina is listed in the EU Habitats Directive as qualifying interest for Special
Area of Conservation (SAC) site designation. Since 2004 the grass is also on the ‘List of
Threatened and Declining Species and Habitats’ drawn up by the OSPAR Commission.
Designation: Almost all of the known sea grass beds in Ireland are located in protected
sites. However official Z marina records did not include Lough Foyle as a bed location
and the species rich outer estuary is not designated as Natura 2000 site under the
Habitats or Birds Directive.
Problems of ownership and responsibility for biodiversity and Nature law enforcement
in this cross border site would pose a problem if the sublittoral was protected.
Recent designations of Marine Protected Areas (MPA) under the OSPAR Convention
would also have been a feasible way of increasing protection., but this was carried out
without consultation and did not include this site.
1.2 Coastwatch Survey work and Results
The Western shores and shallow water of Lough Foyle were checked for Zostera seagrass from Quigley’s point in the mid lough northwards to Inishowen Head at the open sea. The
aim was to locate sea grass patches and beds; and if found, take simple recordings as a first
citizen science scoping exercise. The survey took place on suitable spring low tide days
between May and October 2012. A team of local divers and Coastwatchers with traditional
knowledge of the area participated, initially coordinated by Karin Dubsky, later by the local
Foyle Coastwatch group. Further records were gathered, during the autumn 2012
Coastwatch survey where blades of sea grass swept up and visible landward edges at spring
low water were noted.
Summary of Results: A string of long Seagrass Zostera marina beds were found straddling
the Lough Foyle Donegal coast from the Salt Pans (south of Carrickarory Pier) to north of
Glenburnie Bay just short of Greencastle, while blades of dislodged grass blades and
rhizomes were found in the same survey area, as well as further south at Quigley’s Point
and on Shroove beach at the open sea. Typically beds were 30-80 m wide and up to 500m
long occupying a band of sandy to sandy mud shell sediment, between ~0.4 and 1.4m deep
at mean low tide.
Limiting factors: The upper and lower limits appeared to be determined by depth of water,
as the sediment remained suitable above and below the band occupied. Plant exposure at
lowest spring tides may be causing desiccation and so determining the landward shallow
water limits, while deeper water may pose a transparency problem for photosynthesis.
Plant health varied from excellent quality with lush, dense clean dark green exceptional
long blades (some > 1.5m) in the seaward facing shelter of the old Moville pier, to healthy
looking ~ 0.5 – 0.8m high swart in the majority of the beds and patches, down to thin, part
discoloured plants due to massive epiphyte cover converging into Ectocarpus veils between
plants in worst hit areas. Bright green shoots growing among these were short and young,
but did indicate potential for restoration. The worst hit areas were the beds in the shallow
bay north of Moville around Carnagarve beach. Bed quality improved again further north
from Glenburnie.
Biota: This initial scoping survey did not include scientific biota recording. However it was
noted – especially in the May and July surveys that before even reaching the beds and once
the boat was above a bed that the seaward limit was full of small fish. Juvenile fish also
flushed from grass beds when approached from shore side.
Comparison with past records: Beds which were found occupied the same general locations
as pointed out by fishermen, divers, a lough pilot and elderly people asked to remember
the grass from swimming or boating in the area. Though several recollections suggest the
beds extended further seawards around and were larger in the past. Several locals
independently pointed out further beds which used to be in the Moville area, but though
the locations were well searched no trace of these beds were found.
Nutrient inputs: The area was searched for obvious local high nutrient level discharges
around the poor quality Carnagarve bed area, but there were neither high nitrate values in
discharge points, nor areas of extra lush grass in the hinterland, which could point to
episodes of high local nutrient input (e.g. excessive slurry spreading). However the Moville
raw sewage outfall plume was seen to move along shore on an outgoing tide in the seagrass
band line on one calm July day site research visit. Any light onshore wind would ensure the
fresh water plume would stay in the bay and feed seagrass and filamentous nutrients
released may be responsible for the observed seagrass bed eutrophication.
Survey Conclusions:
Parts of the Lough Foyle Donegal coast still contain extensive Zostera marina seagrass beds.
However going by local knowledge the range and hectares covered have reduced
significantly in the last few decades. Zostera grass was missing at Moville peer and just
north, with first location of the next bed north in poor condition with part smothered plants,
matching the descriptions of eutrophication impacts in the literature.
Given the value of the habitat it would be important to commence a more detailed
monitoring of this coastal area. Resources would be well spent if this could be done in
cooperation with the local committed public who all worked as volunteers on the scoping
survey. Further it is urgent to tackle the known source of nutrient enrichment – raw sewage
discharge at Moville peer and ensure that no other sources contribute.
____________
Goals of Sewage Treatment: The primary goals of planning waste water treatment and
investing in the infrastructure is to remove the source of nutrient enrichment which is
damaging biota and coastal habitats and source of microbiological water pollution, threating
consumers of edible shellfish and swimmers in summer. Added impetus to carry out the
work is provided as it is urgent to become compliant with EU and national water legislation
and to encourage tourism activity.
Sewage Treatment Plant proposal and impacts of the discharge pipe
2.1 Construction phase:
The pipe is to be set into the intertidal substrate which requires dredging an
underwater trench, or digging it at low tide. As the substrate is a coarse sand/shell
mix and the ground water table is near the surface, the trench wall gradient will be
a v shaped.
The method described in the EIS ( section 4.3) is:
The installation of the treated effluent outfall pipeline will be by open trenching
overland (including the intertidal area), to a depth of 1m. However, once past
the low water level, the pipe will be laid directly on the Lough bed.
The trench dimensions are given (section 5.2) ‘A pipe trench measuring approx. 1m in width
and 1.5m in depth will be excavated to accommodate the outfall pipe.
Finally the pipe is to be on the seabed – which is to be levelled – once it enters the
sublittoral. No information as to securing the pipe on the sea bed parallel to strong
tidal currents is provided.
The Natura map copied below shows the proposed path of the pipe from land through the
shore. The sea grass bed is roughly delineated by wide stripes rather than an actual bed
edge.
Concern with EIS data:


The trench dimensions are unrealistic for intertidal sandy shell sediment.
If the pipe is to be buried in a trench 1.5, deep then the trench width at the intertidal
surface would have to be at least twice that is 3 m. Wet intertidal sand would not
hold, but rather slump back into the 1.5 m deep hole.
After the pipe comes to the surface in the sublittoral the ground is to be levelled.
There is no mention of extend, or what that would do the seagrass bed.
2.2 An estimate of direct loss:
An approx.. 60 m length of the sublittoral seagrass bed is to be crossed by the discharge
pipe after the bed is levelled. If that laying and levelling involved a 4 m wide band - seen as
very conservative going by marine discharge pipes laid in other sites - it would cause loss of
approx. 240 sq m of Zostera marina habitat.
2.3 Indirect loss
Sediment plumes from trench digging and infill as well as bed levelling during construction
are certain to move into the Zostera bed, as it forms a curtain around the beach and the
construction is to go through that curtain. This will cause siltation and increase in water
turbidity. Reeduced water transparency impedes photosynthesis.
A review of Z marina sensitively to these and other factors was carried out in Plymouth
marine lab and used by Dr Harvey Tyler-Walters (20081) to create a sensitively key
copied below. This is interactive if viewed electronically and continues to chemical factors
not included here.
Click factor name to
view rationale
Intolerance
Recoverability
Sensitivity
Substratum Loss
High
Very High
Smothering
High
Very High
Increase in suspended
sediment
Intermediate
Moderate
Moderate
Desiccation
Intermediate
High
Low
Increase in emergence
regime
Intermediate
High
Low
Increase in water flow
rate
Intermediate
Moderate
Moderate
Zostera marina. Common eelgrass. Marine Life Information Network: Biology and
Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological
Association of the United Kingdom. [cited 05/11/2012]. Available from:
http://www.marlin.ac.uk/reproduction.php?speciesID=4600
1
Increase in
temperature
Tolerant
Not relevant
Not sensitive
Increase in turbidity
High
Very High
Increase in wave
exposure
High
Very High
Noise
Tolerant
Not relevant
Not sensitive
Visual Presence
Tolerant
Not relevant
Not sensitive
Intermediate
Moderate
Moderate
High
Low
High
Abrasion & physical
disturbance
Displacement
Irish recent example of observed impacts:
In summer 2011 Coastwatch surveyors noted areas of Zostera marina stress and dieback in
Tramore Backstrand Co Waterford. The tidal channel in which the seagrass was growing
had been exposed to silt plumes from lagoon works 300 m away. A second channel not
exposed to the silt load continued to have healthy seagrass.
3.Recovery from the construction phase
When sediment is levelled and a pipe laid onto it, the resulting surface and local
hydrology will change. The gap created may also change local currents. We have virtually
no experience of such post construction Z marina bed restoration in Ireland and very little
in Europe. It cannot be assumed that this bed will repair. It is more likely that plants will not
be able to recolonize the newly made ground for some time.
4. Sewage treatment operation phase:
Although the sewage discharge will have received secondary treatment it will still entail
the release of warm nutrient and chemical enriched fresh water.
Zostera marina is sensitive to heat, fresh water and nutrient enrichment. It is therefore
of critical importance to model how often the discharge plume would travel under
different tide and wind directions into the seagrass beds which are running behind and
either side of the discharge point
The EIS did not provide this information. It did not even map the sea grass beyond the
immediate outfall line.
5. Sewage treatment emergency overflows:
Emergency situations occur and the predictable risk needs to be taken into account. The
causes include malfunction of pumping station, blockage and toxic pollutants entering the
sewer and sewage treatment system, causing the microbs which treat the sewage to die off
on mass. Part or untreated sewage with or without added chemical contamination can
then be discharged.
If raw discharge with sanitary material debris is the problem then this can be caught in the
zostera bed between plants and remain there for some time. Apart from smothering and
high nutrient levels aiding opportunistic epiphyte growth it is aesthetically extremely off
putting.
If the problem is a release of chemicals – eg Creosote detected in Rings end on several
occasions, then the discharge could both kill or damage the Zostera bed and the species
rich biota associated with it.
Pipe Damage: One feature common to most sea outfalls is that they have a limited life
span. In this case the pipe is only protected while on land and the intertidal. Once below
low water, it is to be free standing, anchored to the sea bed, with currents running
perpendicular to the pipe. A similar discharge pipe set into the North Wexford coast at Tara
Glen in 2008 was lifted by storms in winter 2010 and is free floating since then.
Oyster The area between sea grass and mouth of the discharge used to support a native
oyster – Ostrea edulis bed until 15 – 20 years ago ( Whiskey Rock fishermen including Paul
Mc Laughlin personal communication) While today only empty shells remain , this too –
like the sea grass bed could have been effected by the Moville raw sewage outfall and
would have the potential for restoration once the source of nutrient enrichment was
removed. Especially as native oyster stock is still found in the lough and the species is now
red listed for protection and a species action plan has been drawn up in Northern Ireland.
In L Ryan UK, a planned sewage treatment plant discharge pipe was diverted from the lough
to the open sea after risks of pollution impacts in case of malfunction were deemed too
high. (Tristan Hugh Jones 2011)
6. Conclusions
A sewage treatment plant is needed and well overdue for this area of the Inishowen
peninsula. It will control and treat sewage to reduce the loading with nutrients, organic
matter and chemicals.
However the discharge pipe location planned is not suitable, given:
-
the high biodiversity value of the area
and knowing
the damage which is certain to occur in construction,
the lack of sea grass restoration experience
the risk associated with malfunctioning incidents
7. Recommendations
Reconsider the discharge from the planned treatment plant to ensure the discharge
is located in a suitable but least sensitive location, away from valuable and
threatened sea grass beds and prime restorable native oyster ground .
A focussed completion of started research of the second alternative location at
Green Castle harbour may provide a fast efficient solution to the problem.
Some work has already been done on this route - such as the archaeological
survey. From our scoping survey there is no sea grass around that busy harbour
area.
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Acknowledgements
I wish to thank all Donegal and Foyle Coastwatch group members who have done the
extensive sea grass fieldwork gratis, as well as Trish Murphy for commenting on the report
summary used here.