Attachment B.2 Characteristics and Composition of the Substance Or Material for Disposal

POC Maintenance Dredging Application | January 2014

Attachment B.2
Characteristics and Composition of the
Substance or Material for Disposal

Reports included as part of this attachment:

•

Assessment of Benthic and Fisheries Impacts of Maintenance Dredging in Lough

Mahon and the Lower River Lee, Aquatic Services Unit (March 2013) [included separately within Attachment B.2]. Relevant references include:

Section 2.2 – Sub-tidal Benthos Survey Section 2.4 – Inter-tidal Survey Results & Overview Section 2.5 – Sub-tidal Surveys

•

Water Injection Dredging Tracer Study, Van Oord (July 2012) [included as part of Attachment D.3]. Relevant references include:

Section 2.2 – Tracer Particle Size Section 3.2 – Sampling Survey Section 4 – Data and Results

Castletownbere Feasibility Report | December 2013

••

June 2011 Sediment Sampling Results [included separately within Attachment B.2]. In summary, these results describe that the sampled material generally comprises black mud with over 50%-60% fraction size less than 63 microns.

June 2013 Sediment Sampling Results [included as part of Attachment B.1 (i)]. In summary, these results describe that the sampled material generally comprises mud with over 60% fraction size less than 63 microns.

Report Reference: IBM0455/R/KG Revision Number: -

2

rpsgroup.com/ireland

Assessment of Benthic and Fisheries Impacts of Maintenance Dredging in Lough Mahon and the
Lower River Lee

(2011-2012)

Commissioned by: Port of Cork Undertaken by: Aquatic Services Unit (UCC)

(March 2013)
1

TABLE OF CONTENTS

SUMMARY

3

7

IINTRODUCTION

Study Outline

7

77899
Receiving Environment Sampling Areas Sampling Frequency and Timing Data Analysis and Study Outputs Feasibility of Achieving the Study Goals

METHODOLOGY & RESULTS – BENTHOS

11

Intertidal Soft Sediment Survey Sub-tidal Soft Benthos Survey Sample Processing Intertidal Survey Results & Overview Community Analysis of Intertidal Data Discussion of Intertidal Results Subtidal Surveys
11 13 16 17 65 66 68 92 92
Community Analysis of Subtidal Data Discussion of Subtidal Data

METHODOLOGY & RESULTS – FISH

94

Overview Timing
94 94
Methods Fisheries Results
94 97
Implications of Fish and Invertebrate Diet for the Dredging in Lough Mahon Type of Dredging and Potential Impacts Overall Conclusion from Fisheries Surveys
121 122 122

REFERENCES

124

Appendix 1: Trawl Results and Maps of Trawl Tracks

126

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SUMMARY Introduction and Brief

As part of a study to assess the potential impacts of dredging within the Lower River Lee and Lough Mahon, the Port of Cork commissioned the Aquatic Services Unit to undertake a study of the intertidal and subtidal infaunal benthos and of the fisheries within the and adjacent to the dredged channel from the City Quays upstream as far as Marino Point at the lower end of Lough Mahon.

Survey Outline and Methods

The survey concentrated on 4 main areas and a control site in the North Channel at Rossmore. Area 1 was between the City Quays and Marina Power Station, Area 2 stretched from the Marina Power Station as Blackrock Castle; Area 3 stretched from Blackrock Castle to approximately half way down Lough Mahon, while Area 4 continued to the end of Lough Mahon to the bend opposite Marino Point

Sampling included intertidal core sampling and sub-tidal grab sampling within the study areas as well as fishing in all the same areas using a range of fishing gear types.

The study comprised a main baseline and a main follow-up survey covering all sampling methods in in all survey areas in May/June 2011 i.e. pre-dredging and again in May/June 2012 about 5-9 months post-dredging. A quarterly survey was undertaken in late February/early March 2-6 months post dredging involving a reduced sampling intensity. Some additional baseline fisheries surveying (confined to beam trawling) was undertaken in late August 2011 just prior to the start of dredging.

Benthic Survey – Methods and Results

Intertidal Macrobenthic Sampling

4 intertidal transects were studied, T1-T3 in the greater Lough Mahon area (T1 by Hop Island, T2 by Carrigrenan, T3 inside Marino and T4 the control site in the North Channel at Rossmore).

At each site, replicate stove-pipe cores were taken at three tidal heights High Shore, Mid-Shore and Low Shore.

Pre-dredging sampling took place in May 2011, and post-dredging took place in February 2012 and June 2012.

The results indicate that there was a pronounced difference between the numbers and biomass of macroinvertebrate infauna between the four transects with T1 and T4 having higher numbers and biomass on all sampling occasions than T2 and T3.

There was also a pronounced difference between the numbers and biomass of invertebrates between shore heights at all transects with the low shore almost always having considerably lower abundances and biomass than the mid or high shore stations and the high shore sites tending to have the highest numbers and biomass at all transects during all three sampling occasions.

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During February 2012, some 2-3 months after the cessation of maintenance dredging in the shipping channel there was a pronounced drop in both faunal numbers and biomass at all transects including the control site (T4). Because this decrease was so clear across all sites including the control and at all tidal heights it was considered to be a normal seasonal trend. Such seasonal drops in biomass and numbers are widely reported in the scientific literature.

During the June follow up survey, some 5-7 months after the cessation of maintenance dredging, macroinvertebrate populations from the February, saw a significant increase across all transects, again this would be expected as a normal seasonal trend on intertidal mudflats. The results varied between transects and between shore heights, so for instance at T1 the biomass at the high and low shore stations increased to the levels recorded in the May 2011 baseline survey, while the mid-shore sites remained as low as during February 2012. At T2, biomass remained the same or increased at all tidal heights compared to the May 2011 baseline and at T3 the low shore biomass did not increase above the levels recorded in February, whereas at the mid-shore and high shore stations they did. Finally at the control site T4 in Rossmore, while stations at all three shore heights increased in biomass compared to the February seasonal low, only the low shore site reached (and exceeded) the values recorded during the May 2011 baseline. This latter result is attributed to finescale patchiness in the distribution of benthic invertebrates.

Finally, when all of the data from all three sampling runs is pooled and analysed using a multivariate analysis technique – MDS (Multi Dimensional Scaling), all the data points (transects, tidal heights and sampling occasions) pool together into a single cluster, which indicates that we are dealing with a single intertidal faunal community typical of sandy mud conditions.

The inter-transect, inter shore height and between sampling run variation noted within the data is concluded to fall within normal temporal and spatial variability ranges typical of such communities with little if any influence from the maintenance dredging operations in the shipping channel.

Subtidal benthic Infauna

Subtidal benthic grabs were taken in four sampling areas (Area 1 to 4) during the study and at three stations across the channel within each of these four locations.

Sampling for benthic infauna was carried out on three occasions, namely May 2011 before maintenance dredging of the channel and in February 2012 and June 2012 after the dredging had taken place.

The results indicate that at virtually all stations within in all four locations there was a drop in infaunal macroinvertebrate biomass, when the May 2011 data is compared with the June 2012 data. On average the reduction in biomass is in the order of around 50%. This change has been attributed to the dredging and was expected. It is notable that localised colonies of the Peacock fan worm (Sabella pavonina) on the un-dredged margins of the channel, do not appear to have been adversely impacted by the dredging, suggesting that the impacts to the macroinvertebrate infauna were largely confined to the channel itself.

Fisheries Surveys, Schedule, Methods and Results

Baseline fisheries surveys were conducted in the four survey areas (Areas 1-4) and at the Rossmore control site in May/June 2011 with a main follow-up survey in May June 2012. In addition, some

4limited additional baseline data was collected in August 2011 just prior to the commencement of dredging while a more extensive quarterly follow up survey was carried out in February 2012.

4 gear types were deployed, baited traps (Areas 1 and 2), fyke nets (Areas 1-4 and the control site), beam trawls Areas 1-4 and Rossmore and seine nets –Rossmore.

The main target of the surveys were the fish species living on and in close proximity to the bottom as it would be expected that these would more likely to be impacted than mid-water (pelagic) species. In addition however the levels of larger mobile epibenthic / hyperbenthic crustaceans were also surveyed, principally Crangon (brown shrimp) and green crab (Carcinus maenas) both of which were widespread and abundant within the study area and both of which are considered important components of the estuarine food web.

Both baited fish traps and fyke nets caught large amounts of green crab in May/June 2011 and again in May/June 2012 at all of the sites where they were deployed. Although biomass was reduced at some stations in 2012 compared to 2011, the data is quite variable between sites and so a definite statement of cause and effect cannot be made.

In addition to crab, both traps and fyke nets caught small numbers of fish including dogfish, cod, pollack, eel, flounder, plaice, bull-rout, hooknose, sand-smelt and most frequently 5-bearded rockling. The small numbers preclude any conclusions being drawn about these fish.

The most intensive aspect of the fisheries survey was the beam-trawling which resulted the capture of 26 species, which when combined with the species 3 species which were only taken in fykes (i.e. lesser spotted dogfish, bull-rout and sand smelt) brings to 29 the number of species recorded during the survey. The majority of these are either classified as Estuarine Species (Nilsson’s pipefish, black goby, common goby) that are strongly tied to the estuarine environment, Marine Migrants (dab, plaice, flounder) which spawn at sea and whose juveniles use the estuarine environment for food and or shelter from predators, or a combination of both (sand goby, hooknose, greater pipefish). Almost half of the species encountered were only noted on one or two occasions, including grey gurnard, whiting, transparent goby and a conger eel larva.

The most widespread and populous species included, sand goby, dab, plaice and hooknose, followed

by flounder, Nilsson’s pipefish and greater pipefish, with black goby also being common in Areas 4

and Rossmore. Common goby was only found in numbers in the intertidal zone of Rossmore where they were taken in a beach seine net.

Trawl returns were slightly lower both in terms of species and numbers of individuals in February 2012, although dab were more widespread and more numerous in February 2012 than during any of the other three sampling runs.

There was no reduction in diversity between the May/June 2011 baseline survey and the May/June 2012 main follow-up survey and all the more common species were all well represented on the latter occasion also. Gut contents analysis undertaken for the study showed a significant proportion of benthic infauna in the diet of species such as juvenile plaice, dab, sand goby, black goby and flounder. Given that the benthic survey indicated a reduction in the biomass of infaunal macroinvertebrates within the dredged channel, one might expect these species to be adversely impacted. However, all of these species consume a range of benthic prey, and a number (including flounder, sand goby and dab in particular) are known to be very opportunistic in their dietary habits and would therefore be expected to shift to other food sources e.g. mobile epibenthic crustaceans

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(e.g. amphipods) and pelagic food items such as calanoid copepods and mysids, thereby dampening the impact of the reduced infaunal prey biomass. Juvenile plaice and to a lesser extent black goby, which appear to be more small polychaete and larger polychaete specialists respectively may be

more susceptible. However, the data from the trawls doesn’t indicate any clear reduction in plaice frequency or numbers between the 2011 and 2012 summer surveys, while data on black goby isn’t

frequent enough to make any definite statement in this regard. It is important to note that in the case of marine spawners such as plaice, dab and flounder for example, the strength of the next annual recruitment of young fish from the plankton will not be impacted by such a small-scale, localised anthropogenic disturbance as the channel dredging in Lough Mahon, i.e. the dredging in any given year will not determine the number of recruits to the population in the following year(s).

Crangon and green crab were by far the dominant mobile epibenthic faunal species captured in trawls and they occurred in all areas. There is some indication from the data that the wet weight biomass of both species in May/June 2012 catches was down in the region of 50% on the baseline data, although the high level of variability in the data makes this a tentative observation only. If the data does indicate a real reduction, then, based on published papers it could be within normal interannual variation in biomass in the case of Crangon at least. If it is due to dredging, it seems unlikely that the overall food web implications would be very significant, given that both species are still very numerous within the dredged channel. Given that both species are known to be predators of small fish including gobies and plaice, a reduction in their density could also have beneficial knock-on effects by reducing predation on juvenile fish. The changes observed however are fairly modest and they are considered unlikely to have very significant impacts within the study area as a whole, especially considering that the dredged channel is bounded by extensive sub-tidal and intertidal banks which remain unaffected by the dredging and which harbour all the same species.

Recommendations

A shortcoming of studies of this type is that they tend to take a snapshot in time, with little if any previous data and follow-up data available to put the findings into context. This is especially problematic when a significant degree of inter-annual variability in numbers and biomass of certain species might be expected. In order to reduce this effect and to assist in distinguishing between variability due to normal inter-annual changes in biological communities and that associated with dredging, it is suggested that limited annual sampling might be considered. Given that the Port Authority will require to undertake maintenance dredging about every three years, it would be useful to carry out limited annual sampling in the intervening years as follows. One of the Area 4 intertidal transects and sub-tidal sampling points could be sampled (once in summer and once in winter). In addition, a 2-m beam trawl could be taken in June each year in Area 3 and Area 4, with a record kept of the species landed and their relative dominance. The size distribution of the Crangon and green crabs landed would also be taken measured from the trawl catch and this would also provide useful contextual data for follow-up studies in the future.

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1

INTRODUCTION

Study Brief

1.1

The Aquatic Services Unit were commissioned by the Port of Cork to assess the impact of the Port’s

Proposed Maintenance Dredging Programme in Cork Harbour on the benthic infauna of the intertidal and subtidal habitats adjoining the shipping channel from the City Quays as far as Marino Point at the seaward end of Lough Mahon. The project also aimed to assess the impacts of the dredging on the mobile epifauna and fish within the same area.

1.2

Study Outline

The project has been designed to get a comprehensive overview of the main components of the fauna within the study area. Benthic macroinvertebrates (mainly oligochaetes and polychaete worms and bivalve molluscs), were sampled in intertidal and sub-tidal areas using cores and grabs respectively, while mobile epibenthic fauna mainly crustaceans, (shrimp and crab) and small fish were sampled using beam trawls, while fish and crustaceans were also targeted in baited pots, fyke nets and beach seine nets, as appropriate.

Benthic macroinvertebrates were identified, counted and their wet-weight biomass measured while fish were identified counted and measured. A selection of fish species, especially the most numerous were retained for gut contents analysis. Mobile epibenthic crustaceans were identified and bulk weighed, with a representative sample retained for size frequency analysis.

The aim of the sampling programme was to assess whether there were any measurable before-after differences in each of the sampled components of the aquatic ecosystem that could be attributed to the dredging. This was to be achieved by direct changes in biomass and community structure as well as an analysis of the food-chain linkage between each component to see whether these had be altered due to the dredging and whether such change had adversely impacted the overall functioning of the estuarine ecosystem within the study area.

1.3

Receiving Environment

The estuary comprises a narrow channel from the City Berths as far as Blackrock Castle and after that widens out into the mud and sandy mud expanses of Lough Mahon. The intertidal area of Lough Mahon comprises mud and sandy mud flats, both north and south of the shipping channel, with narrow rocky belts in the mid to upper intertidal and extensive mussel beds in places e.g. by Carrigrenan Point on Little Island. A bit off the main channel, between Marino Point and Fota, there are extensive intertidal flats, which are used by waterfowl as feeding areas in the appropriate seasons.

1.4

Sampling Areas

The study area was divided up into 4 sampling areas (Area 1 to Area 4) and a control site (see Figure 1). The areas are as follows: Area 1- City berths to the Marina Power Station. Area 2 - Tivoli to Blackrock Castle, Area 3 Upper Lough Mahon from Blackrock Castle to Hop Island and Area 4 - Lower Lough Mahon from Hop Island to Marino Point. The control site was located in the North Channel just south of Rossmore (Figure 1.1).

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1.5

Sampling Frequency and Timing

See Table 1.1 for a summary of the sampling programme as undertaken. A baseline sampling was undertaken in May and June 2011 when the full range of sampling was undertaken in all four study areas and the control, in advance of dredging. As dredging didn’t commence until late August, a short beam-trawl survey was added prior to the commencement of Water Injection Dredging (WID) which was undertaken from the City Berths to below the Tivoli Docks. Due to technical matters, the WID failed to give satisfactory results in the Tivoli Docks area and Trailer Suction Hopper Dredging (TSHD) was repeated in this area and continued on down the main Lough Channel to just beyond the bend at Marino Point. The dredging was undertaken in a number of short campaigns and finally

concluded in early January 2012. The first ‘quarterly’ re-survey was undertaken in late February and

early March and included intertidal and subtidal benthic sampling in all 4 areas and the control and beam trawling in all the same area. Three to four months later from late May to Mid June 2012 i.e. about 8 months since Area 1 and much of Area 2 had been dredged (WID) and about months 6 since Area 3 and 4 had been dredged (TSHD) a full follow-up survey involving the full range of sampling methods was carried out.

Area 1

Area 2

Area 3

Area 4

Control

Baseline Survey (May-June 2011)

Intertidal Benthic Sampling Subtidal Benthic Sampling Baited Traps Fyke Nets Seine Nets xxxxxxxxxxxxxxxxxxx

x

Trawls

x

Additional Sampling (August 2011)

Trawls