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An Assessment of the Trophic Status of
Estuaries and Bays in Ireland

PREPARED FOR THE DEPARTMENT OF THE ENVIRONMENT AND
LOCAL GOVERNMENT

DIE CUT

   

ENVIRONMENTAL PROTECTION AGENCY
An Ghniomhatreacht um Chaomhnü Comhshaoil

BR) An Assessment of the Trophic Status of
Estuaries and Bays in Ireland

PREPARED FOR THE DEPARTMENT OF THE ENVIRONMENT AND
LOCAL GOVERNMENT

March 2001

ENVIRONMENTAL PROTECTION AGENCY
An Ghniomhaireacht um Chaomhnü Comhshaoil
PO Box 3000, Johnstown Castle Estate,

Co. Wexford, Ireland

Telephone: +353-53-60600 Fax: +353-53-60699
Email: info@epa.ie Website: www.epa.ie

CONTENTS

Summary of Conclusions

Introduction
Determining Trophic Status
The Monitoring Programme

Results and Assessment

Annexes

13

ACKNOWLEDGEMENTS

Water quality data for the purposes of this assessment were gathered in a monitoring
programme co-ordinated by the Environmental Protection Agency involving the
Agency’s laboratory at Dun Laoghaire and other laboratories. Information gathered
by the Agency’s laboratories at Castlebar, Kilkenny and Monaghan was gathered as
part of monitoring programmes undertaken for the local authorities in the respective
regions and the contributions of the local authorities to these programmes are
gratefully acknowledged.

In addition, the Agency acknowledges with thanks the following organisations for
contributions of information and other assistance towards the preparation of this
report:

Cork Corporation, Cork County Council, Department of the Environment and Local
Government, Dublin Corporation, Fingal County Council, Kerry County Council,
Limerick County Council, and the Marine Institute.

Maps were prepared by Compass Informatics Ltd.

SUMMARY OF CONCLUSIONS

An assessment has been made of the trophic status of the principal tidal waters in
Ireland receiving inputs of nutrients either directly or viarivers. The assessment is
made for the purposes of the EU Directives on urban waste water treatment and on
nitrates from agricultural sources. The assessment is based mainly on survey data
gathered over the period 1995 to 1999. These data provide for the first time a
sufficient body of information for a formal assessment to be made of the trophic status
of estuaries and bays in Ireland.

A set of quantitative water quality criteria has been developed. The assessment has
been based mainly on the application of these criteria to the survey data. Where
relevant, other factors, such as problems caused by macro algae, have been taken into
account.

Asa result of the assessment the following tidal waters in Ireland are considered to be
eutrophic:

Broadmeadow Estuary Inner
Liffey Estuary

Slaney Estuary Upper
Slaney Estuary Lower
Barrow Estuary

Suir Estuary Upper
‚Lee Estuary/Lough Mahon
Upper Bandon Estuary
Lower Bandon Estuary
Upper Lee Estuary (Tralee)
Upper Feale Estuary
Cashen/Feale Estuary
Killybegs Harbour

The following waters are considered to be potentially eutrophic:

Castletown Estuary

Blackwater Estuary Upper
Blackwater Estuary Lower
Owennacurra Estuary/North Channel

Further consideration of these waters is merited. This will need to take into account
any information emerging on current trends in nutrient inputs.

1 INTRODUCTION
1.1 GENERAL

The objective of this report is to provide an assessment of the trophic status of Ireland’s
estuarine and coastal waters. This assessment is required for the full implementation of EU
Directive 91/271/EEC concerning urban waste water treatment and Directive 91/676/EEC
concerning pollution by nitrates from agricultural sources.

1.2 BACKGROUND

Until recent years there was no national monitoring programme for the monitoring of
estuarine and coastal waters with respect to their trophic status. Studies of water quality and
other aspects of the environment of tidal waters were restricted either geographically or
temporally. The available information was considered to be insufficient to allow an
assessment of the trophic status of Ireland’s estuarine and coastal waters to be undertaken.

To address this information deficit and to provide essential information on water quality for
national purposes, an estuarine and coastal waters monitoring programme was initiated on a
pilot basis during the early to mid 1990s and was intensified thereafter. The bulk of the useful
information is that dating from 1995 onwards.

The primary goal of the monitoring programme was to generate information necessary to
address the specific requirements of these EU water directives in relation to their application
to estuarine and nearshore coastal waters.

Up to the end of 1999, the programme was fully operational for five years. Such a period of
years is essential in order to account for short term variability and to provide a sufficiently
large body of information for assessment purposes. It is considered that the information
generated provides a reasonable basis for the examination of the status of estuarine and
coastal waters in relation to eutrophication.

2. DETERMINING TROPHIC STATUS

2.1 DEFINITIONS OF EUTROPHICATION

The definition of eutrophication as contained in the EU Directive on urban waste water
treatment (91/271/EEC) is as follows:

“eutrophication’ means the enrichment of water by nutrients, especially compounds of
nitrogen and/or phosphorus, causing an accelerated growth of algae and higher forms of
plant life to produce an undesirable disturbance to the balance of organisms present in
the water and to the quality of the water concerned.

The definition of eutrophication as contained in the Directive on nitrates from agricultural
sources (91/676/EEC) is along similar lines:

“eutrophication’ means the enrichment of water by nitrogen compounds, causing an
accelerated growth of algae and higher forms of plant life to produce an undesirable
disturbance to the balance of organisms present in the water and to the quality of the
water concerned.

The definitions recognise the complexity of the linkages between the causes and the responses
of waterbodies to eutrophication. They require that, in order to be categorised as “eutrophic’,
areas of water should have exhibited each of the following:

e enrichment by the stated nutrients, and

®  accelerated growth of algae and higher forms of plants, and

e undesirable disturbance to the balance of organisms present and to the quality ofthe water
concerned.

2.2 GENERAL INDICATORS OF EUTROPHICATION

Eutrophication may be regarded as a change in the state of a waterbody over time in response
to increasing inputs of nutrients. This process may be manifested in some or all of a number
of different features of the waterbody and its ecology, including those listed in Table 2.1
(adapted from GESAMP). However, as demonstrated in the table, the indicators of trophic
status may not be unambiguous in every instance; the process of identifying eutrophication of
a waterbody, and the discrimination of its effects from natural events or the effects of other
forms of disturbance, is not a straightforward task.

Table 2.1 Indicators of Eutrophication and Some Relevant Considerations

  
  

   
 
 
 

It may be difficult to distinguish this from natural
variability. Seasonal factors must be taken into
account.

Discrete phytoplankton “bloom” events may be a
natural feature of the annual cycle in most natural
waters and may also occur more regularly in certain
waterbodies because of their particular character
Natural succession in plant communities may be
difficult to discriminate from changes induced by
eutrophic pressures
There may be problems in achieving levels of
monitoring sufficient to definitively address these
items.

There is not necessarily a link between toxic blooms
and direct nutrient enrichment

Nutrient concentrations
elevated above natural levels

  
    
  
   
 
 
  
    
 

   
  
 

  
  
  
 

Elevated primary productivity,
both of pelagic and attached
plants

 
   
   

 

  
   
 

 

Changes in plant species
composition

  
  

   
    
   
   
  
 

   
  
   
  
 
  
 
  
  
    

 
   
   
 
  
    
 
  
   
  
     
  
  
  
 

Increased duration of blooms
and the occurrence of
unusually dense blooms
Increased occurrence of
blooms of toxin-producing
species
Occurrence of anoxic
conditions due to degradation
of decaying plant biomass
Adverse effects on fish and
invertebrates
Changes in the structure of
biotic assemblages

Impact on amenity and other
human uses of water due to
reduced water clarity,
objectionable odours arıd poor
appearance

 

    
 
 
    
   
    
    
  
 
  
  

 

This can also occur in the presence of excessive
quantities of organic matter from, for example,
effluent discharges
Similar effects may result from natural events and
other forms of human disturbance.

Similar effects may result from natural events and
other forms of human disturbance.

Similar effects may result from natural events and
other forms of human disturbance

    
  
  
   
    

  

 
 
     

Well-established and scientifically rigorous classification paradigms for estuarine and coastal
waters have not yet become available internationally, although this is now being progressed.
At present, therefore, there remains much scope for variance between different approaches to
the task of defining trophic status. It has been necessary, however, to develop a set of criteria
specifically for the purposes of this assessment.

2.3 QUANTITATIVE CRITERIA FOR EUTROPHICATION

This section of the report sets out quantitative criteria for eutrophication in Irish bays and
estuaries and in the immediately adjacent coastal waters. The purpose is to provide a
transparent approach to the assessment of key features of eutrophication. These criteria are

not intended to be used in isolation, as there are aspects of eutrophication as outlined above
for which quantified information cannot be obtained in routine surveys.

International literature has been consulted, as well as available information on experience to
date ofthe international scientific and regulatory communities, in order to develop the list of
criteria necessary for the present assessment. The outcome of this exercise comprises a set of
quantitative criteria.

The criteria are for the purposes of assessing whether or not estuaries are eutrophic
specifically in the context ofthe urban waste water treatment Directive and the nitrates
Directive. In line with the definition of eutrophication in these Directives, there are three
categories of criteria:

(a) criteria for enrichment,
(b) criteria for accelerated growth, and
(c) criteria for “undesirable disturbance’.

As noted earlier, the assessment of eutrophication must take all of these into account.

Estuarine and coastal waters comprise an immense variety of waters in terms of their
bathymetric and oceanographic characteristics. At the upstream extreme, a number of
estuaries have significant upper reaches which are tidal but non-saline, even at times of low
river flow. Such tidal fresh waters (1.e., tidal rivers) are included for consideration in this
document. At the seaward extreme, Ireland’s report as input to the Celtic Seas Quality Status
Report (OSPAR), notes that against a background of large natural variations in nutrient levels
in the Irish Sea it is difficult to resolve anthropogenic influences from natural processes.
Natural processes of upwelling also influence nutrient levels in Atlantic waters offthe south
of Ireland.

Because of the variable characteristics of tidal waters in respect of salinity, the criteria are
defined in terms of three types of waters:

(Ü tidal fresh waters
(ii) intermediate salinity waters (17 psu median)
(ii) full salinity waters (35 psu median)

The criteria determined for use in this assessment are defined in Table 2.2. Linear
interpolation is to be used to establish the criteria for waters between these salinity levels (see
Annex 1), based on the median salinity ofthe waters to be evaluated.

As .noted earlier, it is necessary for such an assessment to be based on a number of years’
data. Also, these criteria do not in themselves provide a fully self-contained means of
classifying waters in relation to trophic status. For example, information on excessive
macroalgal growth and distribution patterns is also relevant. Qualitative observational
information collected on individual surveys, as well as reports from other sources, have
provided support to the assessment of this aspect of the trophic status ofthe waterbodies
under review.

Table 2.2 Definition of Criteria for Eutrophication
in Irish Estuaries and Coastal Waters

Dissolved Inorganic Nitrogen (DIN) (mg/l N)
Tidal Fresh Waters >2.6 median winter or summer

Intermediate waters** >]1.4 median winter or summer
Full salinity waters >0.25* median winter or summer

Orthophosphate (MRP) (ug/l P)

Tidal Fresh Waters >60  median winter or summer
Intermediate waters** >60 median winter or summer
Full salinity waters >40* median winter or summer

Chlorophyli a (mg/m'‘)

Tidal Fresh Waters median summer
90 percentile summer

Intermediate waters** median summer
90 percentile summer

Full salinity waters median summer
90 percentile summer

Dissolved Oxygen (D.O.) (% saturation)
For D.O. there are criteria in respect of both deoxygenation and supersaturation

Tidal Fresh Waters <70 5 percentile summer
>130 95 percentile summer

Intermediate waters** <70 5 percentile summer
>130 95 percentile summer

Full salinity waters ..<80* 5 percentile summer
>120* 95 percentile summer

 

*For high salinity waters, because ofnatural variations, these values are to be regarded as thresholds above
which the issue of possible eutrophication should be considered in respect ofthe areas at the mouths of bays and
estuaries (rather than formal criteria of enrichment or its effects in general coastal waters).

**at median salinity 17 psu

The variation in the criteria over the full range of salinities is presented in Annex 1.