Handbook of Ecological Indicators for Assessment of Ecosystem Health - Chapter 16 potx

University of Aberdeen’s Culterty Field Station located on the banks of the estuary The small spatial extent of the Ythan, especially the estuary, which has facilitated detailed descrip

CHAPTER 16

The Health of Ecosystems: The Ythan Estuary Case Study

D Raffaelli, P White, A Renwick, J Smart, and C Perrings

16.1 INTRODUCTION

The last ten years have seen a call to refocus the sustainability research agendas of the environmental, social, and economic sciences, so that they can be integrated within the overarching concept of ecosystem health (e.g., Costanza

et al., 1992; Rapport et al., 1999) Such integration will not be easy Like all novel approaches, ecosystem health has its critics harping from the sidelines,

as well as having significant conceptual and operational issues to overcome (discussed throughout this volume) One of the most productive ways to present the area of ecosystem health to research scientists and environmental managers alike is through well-documented and understood case studies Here, we describe agriculture-induced changes in an extremely well-documented system, the Ythan Estuary, Aberdeenshire, Scotland, from the early 1960s to date The Ythan is one of the smallest rivers in Europe, but one of the most significant in the present context, due to:

A long history of ecological research on the system carried out continuously over the last 40 years by staff and students at the

University of Aberdeen’s Culterty Field Station located on the banks of the estuary

The small spatial extent of the Ythan, especially the estuary, which has facilitated detailed descriptions of the physical and biological elements, as well as scale-appropriate manipulative experiments for testing hypotheses about function

Detailed historical data on agricultural land-use within the catchment and

on river water quality, available on an annual basis

The U.K national and wider European interest in the Ythan’s nature conservation status and as a test-bed for implementation of European environmental legislation, policy, and restoration

In this chapter, we review the temporal changes in the major physical and biological components of the Ythan Estuary and link these to historical trends

in land use and agricultural practice in the catchment driven by U.K and European agriculture policies We also evaluate changes in the health of the ecosystem using a number of indicators and explore the potential for different policy instruments for restoring ecosystem health in this system

16.1.1 The Physical Context

The Ythan River drains a catchment of approximately 640 km2lying north

of the city of Aberdeen, northeast Scotland The river rises only a few hundred meters in altitude near the Wells of Ythan and runs east to the North Sea coast where it enters the sea at the village of Newburgh Several major tributaries join the Ythan towards the lower reaches, some almost as large as the main river itself, which is never more than a few tens of meters wide, even at the estuary The estuary is basically salt-wedge — that is, with a saline intrusion on the flood tide running beneath a surface wedge of freshwater (Leach, 1971), although wind-induced wave action can easily destratify the vertical salinity gradient because of the estuary’s shallow depths (1 to 5 m) The salinity intrusion reaches approximately 8 km upstream from the estuary mouth The area experiences two tides per day ranging from around 2 m (neap) to around 4 m (spring) The upper and middle reaches of the estuary are characterized by significant areas of mudflat with a high silt content, while the lower reaches are much sandier The lower reaches support extensive intertidal and shallow subtidal mussel (Mytilus edulis) beds Whilst the Ythan is at a relatively high latitude (57200N), the river and estuary are always ice-free, because of the modifying influence

of the North Atlantic drift on this region of Europe A full account of the physical characteristics of the estuary can be found in Baird and Milne (1981)

16.1.2 Long-Term Data Sets

There is a wealth of relevant data on the Ythan (Raffaelli et al., 1999), including:

Annual digests of land use integrated at the level of each of the seven parishes that fall entirely within the catchment boundary; river water

quality measurements made by Culterty staff and the Scottish Environ-mental Protection Agency (SEPA)

Topographic data from charts and maps from local sources, Her Majesty’s Admiralty and the Ordnance Survey of Great Britain

Aerial photography (irregularly)

Biological surveys of macroalgal mats, benthic invertebrates and shorebirds

16.2 CHANGES IN AGRICULTURE

The period after World War II saw dramatic changes in agricultural production across Europe, and these accelerated with the adoption of the Common Agricultural Policy (CAP) in the early 1970s The CAP provided incentives for particular kinds of land use through a series of subsidies and interventions In Aberdeenshire, a field rotation system was still common practice in the 1960s, with oats, spring-sown barley and wheat, potatoes, neaps (swedes), cattle, and sheep, the mainstay of farming The next 10 to 20 years saw major changes in land use within the Ythan parishes (Figure 16.1) These changes reflect:

1 The preferential growing of subsidized cereals, such as wheat and barley,

at the expense of the more traditional oats

2 The introduction of novel crops, such as oil-seed rape

3 An increase in the total land area under fertilizer-hungry cereals and rape, at the expense of grassland, especially rough grassland

4 A shift towards winter- and autumn-sown cereals, such that land is tilled

at a time of high precipitation and run-off

5 An increase in pig production

In addition, intensive agriculture now extends right to the edge of the river bank over large sections of the water course, creating significant soil erosion at many locations Over this 40-year period, application of chemical fertilizer, especially nitrogen-based, increased by a factor of two to three (Domberg et al.,

1998, 2000), independently of the spreading of livestock waste to the land

In addition, about 95% of the land in the catchment is under agriculture, much

of it arable

16.3 CHANGES IN WATER QUALITY

Land-use changes are unambiguously reflected in the water quality of the Ythan River over the same period (Figure 16.2) Since 1958, there has been a two- to three-fold increase in the concentration of total oxidized nitrogen (almost entirely nitrate) in the river water (Figure 16.2a) and a similar pattern

is seen within the estuary, once dilution by seawater is accounted for (Figure 16.2b) Above the town of Ellon, representing the tidal limit of the system and where salinities are typically less than 2 psu, the total load of

nitrogen entering the estuary from the river has increased by a similar factor (Figure 16.3) The only sewage-associated nutrient inputs to the Ythan are at small towns of Ellon (population 15,000) and Newburgh (population 1500) These inputs are negligible in terms of the overall nutrient budget (Raffaelli et al., 1989) There is negligible import of nitrogen from the sea (Figure 16.2b)

The emphasis above has been on nitrogen rather than phosphorus, because while phosphorus is a limiting nutrient for plant growth in freshwater,

it is much less important than nitrogen in marine and estuarine systems In addition, phosphorus shows at best only a weak temporal signal (Figure 16.3) However, it should be noted that the significance of phosphorus remains a lively and contentious issue in this debate, probably because sewage works at Ellon contributes about half of the estuarine phosphorus budget

Figure 16.1 Land-use change in the Ythan catchment over a 30-year period Data from

Scottish Records Office, after Raffaelli (1997).

16.4 CHANGES IN BIOLOGY

The biology of the tributaries and the main river has changed consistent with a picture of nutrient increase (see below) In addition to the routine invertebrate monitoring carried out by SEPA, macrophyte surveys reveal evidence of eutrophication in several stretches, filamentous algal growth is an increasingly common feature of the stream bed and spawning areas (redds) for salmonids have become silted up and hence unsuitable

By far the best-documented changes in biology are for the estuary itself The area of intertidal flat covered by mats of opportunistic macroalgae (Enteromorpha1, Ulva and Chaetomorpha) has increased substantially 1

Molecular studies of the genus Enteromorpha on the Ythan has recently lead to its revision into the genus Ulva (Maggs et al., 2003) However, to retain consistency with previous texts in this review, and to avoid confusion with the Ulva described in those texts, the name Enteromorpha is used here.

Figure 16.2 Nitrogen (mostly nitrate) recorded: (a) in the Ythan River over a 40-year period by

SEPA (after Raffaelli et al., 1989); (b) in the Ythan Estuary pre- and posteutrophication (1960s and 1990s, respectively) Data from Leach (1971) and Balls et al (1995), after Raffaelli (1999).

(Figure 16.4) so that now around 40% of the intertidal area is regularly covered by mats of biomass in excess of 1 kg m1 Nitrogen is implicated in this algal growth (Fletcher, 1996; Raffaelli et al., 1998) and other competing explanations, such as changes in estuarine topography and hydrography, have been evaluated and discounted (Raffaelli et al., 1999)

Algal mats have a major impact on the underlying sediment invertebrate communities The most important is a decline in the amphipod Corophium volutatorand an increase in the opportunistic polychaete worm Capitella sp., although many other taxa are also affected (Raffaelli, 2000) These effects are dose-dependent (algal biomass) and their causality has been confirmed through a series of manipulative field experiments (Figure 16.5; Hull, 1987; Raffaelli et al., 1998, 2000) Algal mats create a hostile physical and

Figure 16.3 Nutrient loads (kg/day) in the Ythan River upstream of the town of Ellon

and therefore representing run-off from the catchment, 95% of which is agricultural Data from SEPA and Leach (1971), after Raffaelli et al (1989).

redox environment for Corophium, which can be exploited by the more tolerant Capitella The effects on Corophium are dramatic, with densities reduced from around 60,000 m1to zero (Figure 16.5) Corophium is the main species in the diet of many of the Ythan’s shrimp, crabs, fish, and shorebirds, including eider chicks (Somateria mollissima), a flagship conservation species The increase in Capitella in areas affected by algal mats does not compensate for the decline in Corophium because this polychaete is not preferred by consumers and is protected by the presence of the overlying mats anyway

Figure 16.5 The response of Corophium and Capitella to controlled addition of algal biomass

in a field experiment carried out on the Ythan Estuary by Hull (1987) A series of similar experiments have all confirmed these effects (Raffaelli, 2000).

Figure 16.4 Coverage of intertidal area by macroalgal mats (Enteromorpha, Chaetomorpha

and Ulva) digitized from aerial photographs taken during the late summer when the bloom is at its maximum Only macroalgal biomass in excess of 1 kg ww/m2 can be detected and the contribution from individual algal taxa cannot be separated After Raffaelli et al (1999).

The decline in the prey base for shorebirds is reflected in changes in the distribution and abundance of wildfowl and waders on the Ythan (Raffaelli

et al., 1999) Redshank (Tringa totanus) and shelduck (Tadorna tadorna) appear to have shifted their distributions to upstream and downstream areas less affected by algal mats Analysis of shorebird counts (Figure 16.6) reveal an initial increase at the start of the eutrophication period, consistent with an overall increase on the carrying capacity of the system (areas not affected by algal growth became more productive through enrichment), followed by a decline over time as these once algal-free and productive areas are reduced

in extent through continued algal spread (Figure 16.6)

Figure 16.6 Shorebird counts, Ythan Estuary The counts are the means of the period from

December to February After Raffaelli et al (1999).

16.5 MEASURES OF ECOSYSTEM HEALTH

A number of measures of ecosystem health have been made on the Ythan, partly in response to an awareness of the nutrient enrichment issue, but also because agencies such as SEPA have statutory responsibilities to carry out such monitoring

16.5.1 Water Quality Index (WQI)

The SEPA Water Quality Index (Scottish Development Department, 1976) integrates measurements of around 15 chemical determinants over a scale of zero (poorest quality) to 100 (best quality) Data collected from 1980 to 1991 show a very slight decline in quality over that period (only a few points on the

Figure 16.6 Continued.

scale) and a more pronounced decline downstream, reflecting the progressively greater contribution of nitrate nitrogen (Pugh, 1997)

16.5.2 Macroinvertebrate Indices of Water Quality

As for many other parts of the world, various biotic indices exist for U.K rivers which integrate score measures of abundance of the benthic invertebrates taken by kick sample For northeast Scotland, the Biological Monitoring Working Party (BMWP, 1978) scoring method has been adopted by SEPA Data from 1982 to 1991 (Pugh, 1997) reveal the following For the main river, there is a reduction in water quality downstream, reflecting increasing inputs as the catchment area increases, with a further depression in the BMWP scores downstream of the main settlement of Ellon, probably due to localized sewage inputs In addition, there is evidence of depression of water quality due to agricultural inputs in the major primary feeder streams (Pugh, 1997)

16.5.3 Estuary Quality Indicators

Estuary water quality in the U.K is classified as ‘‘good,’’ ‘‘fair,’’ ‘‘poor,’’ or

‘‘bad,’’ based on the U.K National Water Council Scheme This scheme is the summation of scores for biological quality (presence of migratory fish, healthy benthic communities), esthetic quality (evidence of esthetic pollution) and chemical quality (dissolved oxygen concentration) The scores for the Ythan pre- and posteutrophication are shown in Table 16.1 Two features emerge from this analysis: (1) the index is relatively insensitive to major changes in benthic community structure (range available, 0 to 2), which is the most noticeable ecological impact on the Ythan Estuary; and (2) esthetic changes have a much greater effect on the score total than biological changes

16.5.4 Ecosystem Indicators

A number of indicators of ecosystem health derive from ecological and ecosystem theory and several of these have been explored for the Ythan

Table 16.1 Estuary water quality in the Ythan, pre- and posteutrophication (1970s and 1990s, respectively) Data from personal observations and various sources

Biology (migratory fish, salmon and flounder, range 0–2) 2 2

*copper originating from pig food.

**PCBs detected by Plymouth Marine Laboratory Mussel Watch program.