Dale Fort Blog Number 33

12 06 2014

Rocky Shore Monitoring at

Dale Fort Field Centre 1996 – 2014

Part 1:  The continuing story of Pelvetia canaliculata and the rough winkles

Introduction                                                                                                                                                            Dale Fort is a marine field centre located near the entrance to Milford Haven. It has operated as part of an educational charity, The Field Studies Council since 1947.   It was the world’s first marine field centre and quickly gained an international reputation for high quality environmental education and scientific research. It retains this reputation to this day. Its mission is to promote environmental understanding for all and its activities are heavily dependent upon the high quality of the marine environment that surrounds it.                                                                                                                                   The work reported on here began in February 1996 as a response to the Sea Empress oil disaster.                                                                                                                                                                         At around 2030 GMT on February 15th 1996, the tanker Sea Empress lost way and crashed into rocks near St Ann’s Head. Several attempts at salvage failed to prevent about 75,000 tonnes of light Saudi Arabian crude oil and about 5000 tonnes of heavy bunker oil being spilled into the sea. The resultant damage to local seashores had profound implications for Dale Fort and seriously affected its work.

Pelvetia with and without oil


The spill inspired the present author to begin the programme of seashore monitoring that continues to this day.

The Sites

Three sites near Dale Fort were selected for study. Intertidal transects were located at:

Dale Fort Jetty Beach,

Castle Beach,


Dale Fort Jetty and Castle Beaches were selected because of their close proximity to Dale Fort (located either side of Dale Point) and their vital importance to the work of the field centre. Monkhaven was selected as a more exposed site located opposite Dale Fort on the north side of The Haven.

Transect locations


The Jetty Beach and Monkhaven transects were surveyed using 0.25m2 frame quadrats at 1m height intervals. The Castle Beach transect was sampled at 0.5m height intervals.

Height intervals were measured using a simple optical level.

Quadrat locations were marked on the rock using yellow Humbrol enamel paint which was replaced over the years by brass screws inserted into the rock.

Where possible, direct counts were made of numbers of individuals of each species present within the quadrat frame.

For all algae, lichens, small barnacles and colonial animals (eg.sponges), percentage cover within the quadrat frame was estimated.

For Littorina saxatilis (agg.) (rough periwinkles) and Melarhaphe neritoides (small periwinkle) the quadrat frame was sub-divided to form a smaller quadrat 10% of its total area. Numbers of individuals were counted within this smaller quadrat and the total multiplied by 10 to give an estimate of numbers per 0.25m2.

All quadrats were photographed.


Selected Results

Multivariate Analysis

The whole data set from 1996 to 2010 was analysed using Plymouth Routines in Marine Ecological Research (PRIMER) multivariate analysis software. This uses the Bray and Curtis coefficient to place sites and species on a two-dimensional diagram. The distances between entities represent differences in species distributions and abundance over time. This type of analysis is extremely useful for large data sets collected in the absence of strong environmental gradients. In the present case, the data were obtained from transects up the dramatic ecotone resulting from the rise and fall of the tides.

primer output monk

The figure above shows the results for the Monkhaven transect over a 15 year period. The clusters represent shore zones for each year and confirm seashore zonation. Zonation is a well known phenomenon on seashores. Any subtle differences that might represent important changes to the shore ecosystem have been lost in the transformation of the whole data set; overwhelmed by the huge effect of height up the shore. The lower shore sites (green triangles) are slightly more diffuse than the other shore zones. This might be expected given the shorter emersion times and less demanding physical environment of this zone compared with the others. We might learn more about temporal, spatial and qualitative changes by investigating individual species or groups of species and sites in simpler ways.

Some Observations on Individual Species


Pelvetia canaliculata at Dale Fort Jetty Beach

Pelvetia canaliculata (channelled wrack) is a brown seaweed that typically lives on the upper parts of sheltered rocky shores all around the coast of Britain. It is remarkable for its ability to withstand desiccation (up to 95% water loss) and is the only seaweed that actually requires a period of drying out. It persists for years and in mature form is large, obvious and unmistakable. These properties render it a useful species for inter-tidal monitoring.

PelvetiaPelvetia at Jetty 1996 - 2014 with legend

P. canaliculata occupied very little of Site 3 at Dale Fort Jetty beach for the first 9 years of the monitoring period. Note that there were no noticeable effects of the oil spill in 1996. This species specializes in survival in a difficult physical environment and oil seems to have been just another physical stress that it coped with. In 2005 it began to increase, until by 2009 it was occupying almost half the site. This continued until 2011 when an even more dramatic decline began. Within one year its abundance had dropped to very low levels comparable to pre-2005.

The obvious question was: What caused these changes? There were no discernible differences to the physical structure of the site over the years, so a biotic explanation was sought. The only animals found on Site 3 that might have grazed on P. canaliculata were Littorina saxatilis (the rough winkle) a small, herbivorous snail that dwells mainly on the upper parts of rocky seashores.

nos of l sax jetty beach 1996 -2014

L. saxatilis is a highly mobile species and its density on a transect will be very variable depending on transient factors like the weather. In wet weather they will browse on the open rock and in dry weather they will seek the safety of a moist crevice or other appropriate microhabitat. This renders them more or less visible to the investigator and consequently consistency of counts can be variable. Nevertheless, the figure above provides convincing evidence that grazing by L. saxatilis might be a factor in the changes seen in the abundance of P. canaliculata. The points on the figure above very nearly reciprocate with the points on the graph of % cover of P. canaliculata. If this is a real effect, it seems likely that it is a reflection of grazing pressure preventing re-colonisation by juvenile P. canaliculata as older individuals die off.   L. saxatilis is far more likely to feed on tiny, recently settled immature algae than large mature specimens.

Pelvetia canaliculata at Castle Beach


CB Pelvetia 1996 - 2014

Lit sax castle 1996 - 2014

On the more exposed (South) side of Dale Point at Castle Beach, there was a remarkably similar pattern of abundance of P. canaliculata. The most obvious difference being that the increase in abundance began in 2006, one year after the increase seen at Dale Fort Jetty Beach. The pattern of development continued to mirror that of Dale Fort Jetty Beach up to and including the corresponding decline that also began one year after the decline at Dale Fort Jetty. The reasons for this delay of one year are unknown.

Numbers of L. saxatilis at Castle Beach were not so convincingly the reciprocal of the abundance of P. canaliculata than were the numbers of the species at Dale Fort Jetty Beach.   There were however generally larger numbers of winkles corresponding with less cover of P. canaliculata.

That’s enough of that for now.  This story will continue in a future blog, where we will consider species diversity,  limpets and purple top shells.