Biodiversity Impacts on an Intertidal Rocky Shore

BL6021

Patrick Cross: 114221921

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Introduction

Description of site

The site in question, Garrettstown Beach, is located in the Courtmacsherry Bay coastal water body, approximately 2.5 km south of Ballinspittle village and 8 km

south west of Kinsale, in West Cork, Ireland. This south facing, meso-tidal site is a gently sloping, sandy beach, flanked on both sides by rocky cliffs, with a tidal range of between 2-4m. There are exposed rock platforms at either end of the strand,

comprising of mudstone and sandstone (O’ Connor, 2010). The Ballinspittle River discharges onto the eastern end of the beach through a man-made channel in the

rocky cliff. The catchment consists mostly of dry grasslands north of the beach, which rise to a height of approx. 155 m at the Northern. Agriculture is the

predominant land use in the catchment and comprises a mixture of intensive grassland & tillage farming. The average daily visitor numbers to the beach during

the bathing season is estimated at approximately 2,500, with maximum numbers of visitors per day estimated at 5,000 (Cork County Council, 2010).

Description of problem

Figure 1 shows two possible routes for a sewerage outflow on the rock platform at the Western end of Garretstown Beach. The ecological implications of both will be

investigated and a decision made on the more suitable option.

Increasing cumulative impacts of human populations in coastal areas are leading to broad degradation of ecosystems all around the world (Halpern et al., 2008; Diez et

al, 2014). Many rocky shores are subjected to a variety of stresses caused by human activities, adding to those caused by natural environmental factors such as

emersion in air due to the tides and wave action (Crowe et al, 2000) The eutrophication of coastal waters, caused by increased nutrient enrichment, is a key

driver of change in aquatic ecosystems globally (MEA, 2005). The chronic influx of excessive nutrients originates mainly from anthropogenic sources, particularly

sewage outfalls (Crowe et al, 2000; Costanzo et al, 2001; O’Connor, 2013). The presence of sewage outfalls can alter the temperature, salinity, nutrient and heavy

metal concentration of the surrounding water column (O’Connor, 2013). This, in turn, can lead to temporal and spatial variation of many species, or reduced

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diversity of intertidal species with the dominance of a few algal species (O’Connor, 2013). On wave exposed rocky shores, the effects of sewage effluent are usually

localised to within about a hundred metres of the outfall (Crowe et al, 2000).

Figure 1. Location of the South (orange) and East (blue) transects lines (map source – Google)

In general, it is agreed that the influence of sewerage and an excess nutrient load in water bodies can lead to decreased species diversity, with an abundance of certain

species tolerant of the changed environment. Past studies on the effects of sewage outfall on rocky shores have shown a reduction in species numbers and diversity

(Littler & Murray, 1975). In particular, the replacement of dominant plant and animal species with cyanobacteria, diatoms, green algae omnivores and suspension feeders has been noted internationally (Littler & Murray, 1975; Fairweather, 1990;

López-Gappa et al, 1990; Worm et al., 1999). At different sites on UK and Irish shores it was reported that micro-grazer numbers increased as a result of nutrient

enrichment while there was no change in algal biomass (Wooton et al., 1996). Brown and red seaweeds decreased in diversity with a corresponding increase in

the numbers of green and blue-green algae (Hardy et al, 1993), and a study by O’Connor (2013) revealed weak evidence that benthic assemblages at locations

with sewage outfalls had greater abundances of mussels, limpets and green algae and less fucoid and red algal species compared to shores without sewage outfalls

(O’Connor, 2013).

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Methodology

The following steps were taken to collect and process required data to assess the best route for a sewerage outfall. The class was split in two with four members to a

group. Each group were given quadrats and sheets of species to identify flora and fauna along the transect lines. Starting at the high shore each group ran a transect

line down to the low shore, one in a Southerly direction, the other Easterly (Fig. 1). Four replicate quadrats were recorded at each shore height for each transect

(excluding Southerly high shore where three were recorded). Quadrats were placed randomly and all animal and plant species identified and recorded. To allow

abundance calculations (per m2) plant coverage was estimated as a percentage and animals were counted individually. Two members of a group counted the flora and

fauna present while one identified species and the last recorded the findings. After collection, data was compiled, calculated and graphed using Microsoft Excel, species richness scores added and Shannon-Wiener Indexes scores created using

an online biodiversity Calculator (Young 2015).

Figure 2. Group using quadrat and species list on the Southern transect in the mid shore dominated

by Eliminius modestus and Chthamalus stellatus

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Results

Figure 3. Overall species richness at each shore height on both transects

The species richness graph clearly shows that the Southerly transect supports a greater number of species than the Easterly transect (Fig. 3). This graph also highlights the increase in species variety when moving down the shore profile.

While the species richness measurement provides a level of insight into the assemblages present, it does not describe their relative abundance.

Figure 4. Animal diversity at each shore height on both transects

Figure 4 shows a greater diversity of animal species along the Eastern transect in the mid and low shore but not in the high. The high shores of the Eastern transect

supports Chthamalus stellatus but no other animal species, returning a Shannon-

Wiener Index value of 0.

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Figure 5. Plant diversity at each shore height on both transects

The plant diversity graph shows that the Southerly transect supports a higher biodiversity of plants than the Easterly, particularly in the mid shore. The South

transect mid shore supporting Verrucaria, Laurencia pinnatifida, Ulva lactuca, Lomentaria articulata and Corralina officinalis, with no plant species being recorded

for the East mid shore.

Discussion

Shore zonation patterns (Stephenson et al. 1949) can be seen on both transects but are more clearly defined on the Eastern transect and are controlled by

“environmental factors such as exposure to air, wave action, temperature and wetness/dryness of substrate” (Sorensen 2012). The rising and falling geography of

the Southern transect and the effect of a spring tide (restricting access to the lower shore) may have had an influence on the characterisation of zones. The presence of

Verrucaria at all of the Southerly transects shore heights, compared to only in the

high shore (where It is most commonly found) for the Easterly highlights this issue. Species representative of the sub-littoral fringe were recorded in the lower shore

zones including Helcion pellucidum and Laminariales (Fig. 6).

In line with Stephenson et al. (1949), the species richness increases for both

transects from high to low shore, with the South recording 10, 14, and 18 species respectively and the East 3, 9 and 10. The low species richness count for the East in

the high shore may be attributed to a number of factors including the lower number of rock pools sampled (or present) compared to the Southerly transect (Firth et al.

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2014), and the proliferation of Ulva lactuca or Sea Lettuce (Fig. 7) and its smothering

effect on other species (Ansell et al. 1999).

Figure 6. Helcion pellucidum on kelp

In contrast, the greater richness of animal versus plant species in the East mid

shore (9/0) could be attributed to the prevalence of crustaceans and gastropods in this region, and their effect on the germination/colonisation of algal species

(Jernakoff 2003). The abundance of rock pools on the South low shore may account for the highest recorded species richness score of 18 (Firth et al. 2014) and may

also act as sand traps for the colonisation of Sabellaria alveolata.

While species richness is a measure of the number of species in a community, species diversity combines species richness and their relative abundance (McGinley

2014). Communities with less variation in the abundance of a given species are considered more diverse. In the high and mid shore on the Southern transect, while

both zones support 5 species of plants, the high shore returns a Shannon index of 1.149 and the mid of 0.7953. This result is due to the dominance of Verrucaria over

other plant species recorded. Again, in the South mid and low shore, animal

richness is similar (9/10) while the Shannon results are more varied (0.65/0.99) due to the greater abundance of crustaceans compared to other species in the Balanoid

zone. The Shannon-Wiener Index, as it considers a wider range of variables, should provide a more useful approximation of the biodiversity of an area and the potential

impacts of a proposed development.

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Figure 7. Proliferation of Ulva lactuca on the Eastern transect high shore

Conclusion

Considering the placement of a sewerage outfall on the Western rock platform of Garretstown beach, by a measure of species richness the clear choice is the

Eastern transect. Species richness is markedly higher along the Southern transect. When species diversity is included the decision is less clear. The South clearly

shows greater plant diversity while the East shows greater diversity at the mid and low shore but none at the high shore. The character of the organisms present also

needs to be considered. Plant species are sessile, where the pipe is constructed the species present will be lost. While some of the animal species present are vagile,

they are not actively mobile. Molluscs, crustaceans, worms and anemones will also be lost during the laying of a sewerage pipe.

The length of the pipe is an important factor. As described by the Cork County Council (2010), increases in nutrient loading from sewerage in the water can lead to

an over abundance of sea lettuce, a species that is already prevalent over areas of the study site. While the sewerage pipe will have to span an area of the rock platform its outflow should be placed well offshore to minimise the impact on inter-

tidal communities. Tourism and recreation is a critical factor here also. Garretstown is a popular destination for the public and the introduction of effluent into the

system, if not planned correctly would have a negative impact on the areas recreational capacity. A final consideration is the physical profile of the platform.

The Southerly transect undulates on its way to the water. It would be a simpler

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process to lay a pipe to the East and would afford some level of protection from the prevailing wave energy coming from the South and South West. With all of these

factors considered this author feels the Easterly transect is more suitable, the South showing greater species richness, an equal if not greater level of species diversity

and less suitable terrain for the outflow construction.

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References

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Cork County Council: Environment (2010) Bathing Water Profile [online] Available: www.corkcoco.ie/co/pdf/526195823.pdf [Accessed: 25 March 2015]

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