Portland Harbour Authority's responsibility extends to the safekeeping and preservation of Portland Harbour and its valuable ecosystem. The following excerpts relating to the importance of the harbour's waters was published by the PHA in it's 2006 dated "Portland Harbour Report"  (7/9/06 pp.8-9) and constitute a brief guide for the area. The information here is supported by our own observations in italics. You can download the Full Report together with the Portland Harbour Risk Assessment for further information. 

Project Baseline Portland Harbour

From the Portland Harbour Authority Report:

Portland Harbour represents a semi-artificial deep water tidal basin, enclosed by breakwaters and with tidal exchange limited to three areas through the breakwaters and also at Ferrybridge where the channel links the Harbour with the Fleet.

The tidal range within the Harbour is small and in the order of 1.5-2.3m. The restricted water exchange leads to elevated water temperatures which accounts for the presence of several marine species beyond the typical northern limits of their range.

Tidal flow within the Harbour is generally anti-clockwise, sweeping away from the Fleet, through the Port area and through the South Ship Channel. Given the depth and limited tidal range, the predominant habitat within the Harbour is one that is truly marine. There is a small area of inter-tidal habitat, particularly on the northern and western shores.

In its entirety, the Harbour is an ecosystem of very high importance including a number of nationally important habitats and species.

In addition to the Harbour, the Port land contains areas of significant nature conservation interest.



Statutory and Non-Statutory Nature Conservation Designations

Hamm Beach and the northern shore of Portland Harbour are part of the Portland Harbour Shore Site of Special Scientific Interest (SSSI) which extends along much of the western and northern shores of the Harbour and extending into the Harbour at several locations to the mean low water mark. The northern shore is designated primarily on the basis of geological interest although Hamm Beach and the inter-tidal zone off the beach are primarily of ecological interest. Hamm Beach is also part of the wider Chesil Beach and the Fleet Lagoon candidate Special Area of Conservation (cSAC) although this is still under dispute with the Department of the Environment.

With regard to non-statutory designations, Portland Harbour (along with the Fleet Lagoon) as a whole has been identified as a Sensitive Marine Area (SMA). The SMA designation is a non-statutory designation implemented by English Nature and which is considered to represent the marine area of national ecological importance. In addition, the north-eastern breakwater has been identified as a Site of Nature Conservation Interest. This is primarily on the basis of ornithological interest and particularly Dorset’s largest colony of great black-backed gull. The site also supports a number of notable plants including the nationally rare and endemic rock sea-lavender. Formerly, the breakwater supported a small number of breeding pairs of the internationally protected roseate tern. This species has, however, not bred at Portland since the early 1990’s.


Intertidal Environment

The inter-tidal environment represents a small part of Portland Harbour as a whole and is most noticeable on the northern and western shores. Many of the sands are typically only uncovered on low spring tides. At such times, it has been estimated that the inter-tidal area may extend to approximately 50 hectares although it is generally much less.

The majority of the fauna are suspension feeders, living protected in the sand. Burrowing anemones, peacock and sand-mason worms occur. Bivalve mollusc species are a further feature of this habitat together with the tusk shell variety and two types of predatory snails*. Of particular note is the occurrence of the legally protected and nationally rare lagoon sandworm** which is found at Smallmouth Spit at one of only three sites in the UK.

[*Portland Port's 27/7/2004 dated Oil Spill Contingency Plan (OSCP) - Risk Assessment notes that the bivalve mollusc species of this habitat include Loripes lucinalis and Pandora albida, together with the tusk shell Dentalium vulgare and the predatory snails Mangelia brachystoma and Mangelia nebula. **The same OSCP identifies the nationally rare lagoon sandworm as Armandia cirrhosa and notes that the legally protected lagoon sand shrimp Gammarus insensibilis  had recently been recorded near Portland Castle.]

The middle shore comprises of boulders and stones and provides one of the very few UK sites for the sea slug. On the north-western shore, seaweed covered rocky reefs occur with various species including a local cushion star.

Sand between the reefs is colonised by the nationally scarce eelgrass. These so-called sea grass beds form meadows from approximately Sandsfoot Castle, eastwards along the western ledges and extending into the sub-tidal environment. Sea-grasses are flowering plants which occur in truly marine habitats extending from the inter-tidal zone to the sublittoral region. They are a group of species that are found throughout the world and are particularly abundant in estuaries, bays and lagoons.

The two species which occur in Portland Harbour (and the Fleet) are Zostera ruppia and Zostera marina, both of which are nationally scarce species. Sea-grasses fulfil several very important functions in estuarine and near-shore ecosystems including contributing significantly to the food web, providing shelter and habitat for many organisms and stabilising sediments. Specific to Portland Harbour, the sea-grass beds provide a habitat for the local snails.


Sub-tidal environment

The sub-tidal environment is the dominant marine habitat within Portland Harbour, accounting for approximately 930 hectares of the Harbour as a whole. Whilst the sub-tidal environment may appear to be one of uniformity, this is not the case and a variety of different habitats are found. The harbour entrances provide man-made tidal channels where strong currents and scour occur.

In this habitat, large colonies of alien species of limpet are dominant*. The limpet shells also provide a habitat for many colonizing species including sponges, sea-squirts, and mobile species such as the spider crab. The breakwaters provide a further artificial although interesting and variable habitat**. Along much of their length the breakwaters are flanked by large rough blocks of Portland Stone. This creates a habitat similar to a natural boulder slope. Various habitats are found here according to depth and tidal pattern. Infra-littoral upward facing surfaces of limestone in shallow water are dominated by kelp in the upper infra-littoral zone and red algae at a greater depth. The diversity of the species found in this habitat is generally low.

[* The 2004 Oil Spill Contingency Plan identifies the limpet as Crepidula fornicata, noting the shells provide a habitat for  colonizing species such as Suberites domuncula sponges, sea-squirts such as Phallusia mammillata and mobile species such as the spider crab Maia squinado. **OSCP further states: "The middle shore comprises of boulders and stones and provides one of the very few UK sites for the sea slug Aeolidiella alderi. On the north-western shore seaweed covered rocky reefs occur with various species including the local cushion star Asterina gibbosa."]

In the central regions of the breakwaters, infra-littoral upward facing surfaces of limestone blocks provide sheltered areas with little kelp and dominated by red algae. The rare black-faced blenny is recorded in this habitat. At depths of 6-15m the circa-littoral limestone blocks along the breakwaters are silty, typically species-poor and with few species of algae low abundance of animals. The vertical surfaces of limestone blocks in 0-14m represent a more diverse habitat that is rich in bryozoans, hydroids, ascidians and sponges.

The majority of the remainder of the Harbour is characterised by a deep-water basin experiencing weak currents. Such basins act as traps for silt material and organic debris. This is seen in Portland Harbour, particularly the deeper sections where currents are weak and the depths sufficient that the influence of wave action is negligible. Here, the Harbour bed typically comprises a thick, deep layer of soft organic silt. This habitat, typical of deeper harbours also occurs naturally in some Scottish sea lochs. This similarity is also reflected in the ecology of this habitat.

Specifically, Portland Harbour supports a substantial population of the sea pen. This is a deep-water and very local species more commonly associated with Scottish sea lochs as well as isolated occurrences in other man-made harbours, for example, Holyhead. The colony within Portland Harbour is extensive and represents the only substantial occurrence of the species south of Anglesey.

Leave a comment

Comments are closed.