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Seawater – not as simple as it looks

It’s a typical question you’d get from a child: ‘How salty is the sea?’, and as it’s National Marine Week here in the UK it seems a good one to answer. The simple answer is that there’s around 35 g of salt in every litre of sea water, but that’s only a starting point. Oceanographers are often interested in small differences in salinity, so how else can we express this measurement? Well, a litre of water weighs a kilogram so 35 g amounts to 3.5%, but percentages are too big a unit to be useful, so we use a unit called parts per thousand (‰ or ppt). If we think of percentages as parts per hundred then it’s a straightforward conversion because 3.5% becomes 35‰ – it’s the same relationship as converting a distance in centimetres into one in millimetres. Salinity is now measured using the electrical conductivity of the water and either given as just a number (35) or with a unit called ‘practical salinity units’ (35 psu).

“Coast” by Chris Luczkow is licensed under Creative Commons (CC BY 2.0)
“Coast” by Chris Luczkow is licensed under Creative Commons (CC BY 2.0)

Another question the child might ask is ‘where does the salt come from?’. An obvious source is the rivers, but that’s not quite the whole story. If we look at the composition of seawater we find an interesting characteristic. While the salinity may vary between different locations, the dissolved chemicals that make up that salinity are found in the same proportions. It’s called the constancy of composition. The first seven major components of seawater are, in order, chloride (Cl), sodium (Na+), sulphate (SO42-), magnesium (Mg2+), calcium (Ca2+), potassium (K+) and bicarbonate (HCO3). If we look at typical components of river water we find much less sodium and chloride, and more calcium and bicarbonate, as well as additional dissolved substances such as silicate (SiO2). The difference between river water and sea water is even greater because much of the chloride in river water has come from the oceans via rainfall. So if the river water is the source of the salts in the ocean, why are the proportions so different?

The answer is to do with something called ‘residence time’, which is a measure of how long the element remains in the ocean before being removed. While sodium and chloride flow into the oceans in smaller amounts than other elements of river water, they stay in the ocean for longer. The residence times are also long compared to the time it takes the water to circulate through the oceans, which means that the oceans are well mixed, and this is one of the reasons we have the constancy of composition.

The proportions of the major components are constant but the total salinity can vary, and these small variations in temperature and salinity identify water masses that can be followed by oceanographers. For example, more water flows into the Mediterranean Sea than flows out. This is because a lot of water is lost through evaporation making the remaining water more salty and denser so it sinks. The straits of Gibraltar are relatively shallow compared to the Mediterranean and the Atlantic, so the salty water (called the Mediterranean water) flows out over the straits while lower salinity Atlantic water flows in at the surface.

There’s another important circulation driven by salt. As the Gulf stream crosses the Atlantic it heads north and cools. The remnants of the Gulf stream pass north of the United Kingdom as the Norwegian current. As sea ice forms the remaining water becomes very salty and sinks, forming a water mass called the North Atlantic Deep Water that flows south along the bottom of the ocean all the way to the Antarctic and drives a global pattern of ocean circulation called the thermohaline circulation.

Seawater has a more interesting story to tell than simply the answer to ‘How salty is the sea?’

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Upper Fowey MCZ – what’s there and local opinion

While I was down in Cornwall for the seasearch fish ID course, I went to a public meeting at the Royal Fowey Yacht Club that had been arranged to discuss management of the Upper Fowey and Pont Pill MCZ.

Sailing Boats in Fowey Harbour
Sailing Boats in Fowey Harbour.
CC BY-SA Duncan Greenhill

First of all, a little background. Upper Fowey and Pont Mill MCZ is an unusual marine conservation zone. It’s the second smallest, at around two square kilometres, and despite its small size it’s split into two separate areas. The main part of the MCZ is the upper Fowey estuary and the second area is Pont Pill, which is a smaller estuary that joins the main estuary from the east a short distance inside the entrance to the open sea.

MCZs are designated based on the features (habitats or species) within them. For this particular MCZ, there are six features listed in the designation, and all are habitats. The European eel (Anguilla anguilla) was included in the draft conservation objectives and there was a single record of a long-snouted seahorse (Hippocampus guttaluatus) recorded in the area covered by the MCZ, but the record dated back to the 1960s. Neither of these species were used as a basis for designation. There are areas of seagrass in the estuary but not within the boundaries of the Upper Fowey MCZ, which followed those of the voluntary marine conservation area (vMCA) that was there previously. It may be that there are species and habitats of conservation interest within the estuary, but not within the MCZ and so not currently protected. The six habitats are:

  • Coastal salt marshes and saline reedbeds, which are important habitats for birds and fish, producing a biodiversity ‘hotspot’, as well as providing natural coastal protection. This type of habitat is relatively rare in the south west.
  • Intertidal coarse sediment consists of pebbles, gravels and coarse sand, and is only found at a few scattered sites in the UK. The unstable nature of the sediment means that few animals can live here successfully, with sandhoppers being one of the exceptions.
  • Intertidal mud is what we normally think of when we think of estuaries – the typical mudflat that supports large populations of worms and bivalves.
  • Low energy intertidal rock are areas that are sheltered from wave action and subject to weak tidal currents, which means that seaweeds can flourish, providing shelter and protection and acting as nursery grounds for juvenile fish.
  • The fifth type of habitat is estuarine rocky habitat. Stable rock is rare within estuaries (because muds tend to dominate) and the rocky shore communities can differ quite substantially from those of normal coastlines because of the brackish water and sediment inflow from the rivers.
  • The final type of habitat is sheltered muddy gravels. These are found in areas that are not exposed to strong tidal streams or strong wave action, and the communities of animals found within them depends on the salinity. Fully marine examples of these habitats are scarce in the UK, but are found in both the areas that make up this MCZ. This habitat is important for diversity and is rich in species such as tubeworms, burrowing anemones and bivalves.

The last two habitats are the most important, and are listed as features of conservation importance (FOCI) for this site, which means that they are “rare, threatened or declining“.

Rob Seebold, who’s a marine adviser with Natural England and Sam Davies from Cornwall IFCA ran the meeting. Rob started with a presentation about MCZs highlighting that the aim for MCZs was to make the marine environment more resilient to change. Those involved in conservation often talk about ‘ecosystem goods and services’, for example, coastal areas provide us with ‘goods’ (fish and shellfish), but also services (intertidal mud protects against erosion by dispersing the energy of waves and currents). It’s the protection and sustainable use of these goods and services that enhances the resilience of the particular marine ecosystem.

There was some concern expressed by some in the audience that they would be prevented from pursuing activities they had always done because they area now had a level of legal protection that it had not had before, and whether people coming in from outside the area would ‘play by the rules’. While Rob couldn’t rule out any changes in future he did point out that the features in the MCZ were generally in good condition. The MCZ is regulated by a number of organisations, including IFCA, the Marine Management Organisation, Cornwall Council, the Environment Agency and the Fowey harbour commissioners. The next steps are that the regulators will look at whether further management is necessary and involve local stakeholders if that’s the case, but with the aim of managing features to a ‘favourable condition’ rather than extending the scope of protection. The regulators are also required to report on the status of the sites to DEFRA every six years.

Some of the concern at the meeting related to fishing issues, rather than the conservation zone itself, and Sam Davies from Cornwall IFCA responded to these as part of her presentation. An interesting point related to the bass fishery where the minimum size for landing in the Cornish area is 37.5cm (36cm in the EU), but as a member of the audience pointed out this is below the size at which they reproduce, and that locals were actually pushing for the limit to be raised to 45cm. IFCAs can set minimum sizes within their own areas so long as they are not below the statutory minimum.

It was my first time at a public meeting like this, and I was impressed. The concerns expressed were reasonable and entirely understandable in the local context, and I didn’t hear a single negative comment about marine conservation zones. And that’s important because protection doesn’t succeed through legislation, but because people protect what they value and connect with.

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It’s a fish – day two

Day two of the seasearch course started relatively early (for a Sunday). We met at Towan headland in Newquay by the old lifeboat station, and the car park started to fill with divers in various states of getting ready. Most of the course participants were diving, but three of us (including me) were snorkelling. We accessed the water down a natural rock ramp, which was much less steep than the old lifeboat slipway, and entered the water at 9.30, around two hours after local high water. As I was only wearing a summer 3mm wetsuit the cold shock was a little bracing and left me hyperventilating for a good twenty seconds, as well as giving me the start of a wonderful ‘ice-cream’ headache. Visibility was around 5m as we began to snorkel. We remained on the surface so as not to interfere with the divers surveying below us, which restricted us to observing what was in the water column or on the shallower rocks, and consequently saw mostly sand eels and spider crabs. We were joined by a female grey seal who kept us company for a while before disappearing to visit the divers. As we swam back to the exit point I could see the silhouette of the seal below me, just at the limit of visibility.

Grey Seal
Grey Seal (Halichoerus grypus). CC BY-SA Duncan Greenhill

After coffee and biscuits, we moved across to the other side of Fistral beach as the tide continued to fall to meet Frances and the other participants for the rockpooling session. This was more productive for me personally, catching a large Shanny (Lipophrys pholis, and thanks to Fiona for spotting it), and later a long-spined Sea Scorpion (Taurulus bubalis). The Sea Scorpion was a complete surprise as I ran my hands through the unlikeliest looking crevice in the rock behind where we’d left our bags and found quite a sizeable fish at about 15cm long.

Shanny (Lipophyrys pholis)
Shanny (Lipophyrys pholis). CC BY-SA Duncan Greenhill
Long Spined Sea Scorpion
Long-spined Sea Scorpion (Taurulus bubalis). CC BY-SA Duncan Greenhill

As the tide started to come in we used a seine net to sample over the sand in the surf. It was hard work, and involved coordination so that the top and bottom ropes were hauled in at similar rates, and that the bottom rope was kept low to avoid all the specimens escaping underneath. We found a prawns and shrimps, a juvenile flatfish, and a number of Lesser Weaverfish (Echiichtyhys vipera), which questioned the wisdom of so many swimmers going into the water barefoot. The day ended with pasties on the beach.

Overall, it was a great weekend. I learned a lot, in good company, and hope to return next year to do the seasearch observer course.

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It’s a Fish – Day One

Seasearch is a project involving volunteer sports divers to record the habitats and marine life around the coast of the UK. I recently attended one of their training courses (Fish ID) in Newquay Cornwall. The course was organised by the Seasearch Organiser for Cornwall, Cat Wilding, who’s also the marine survey officer for Cornwall Wildlife Trust, and the tutor was Dr Frances Dipper.

The course was being held at Newquay College and after the usual introductions, the day started with a short presentation where Frances talked about some of the main groups and families and the general characteristics of fish that would be used to identify them. In the second presentation, we moved on the fundamentals – the FLEMMS system. The FLEMMS system is designed so that you can gather a lot of information for identification in what may be a relatively short glimpse as the fish disappears into a clump of weed. FLEMMS stands for:

  • Fins, specifically the unpaired fins. How many dorsal? How many ventral? Is the tail concave, convex or straight?
  • Lateral line. Is it visible in that particular species? If so, is it straight or curved?
  • Eyes. Where are they positioned? Are they large or small? Are they bulging?
  • Mouth. Where is it positioned? Is one of the lips prominent or are the lips equal? Are there any barbels?
  • Markings. Are there any distinctive patterns, colours or spots?
  • Size. Relate the size to something more general: is it finger, hand or arm size?

We then got to practise and started with the easy option – identifying fish from photographs, although as we progressed Frances would mimic the fish disappearing by changing the slides faster. We could either make quick notes about features or make a quick diagram. I chose the diagram method. It starts with a cross to represent the fish onto which we mark the fins, lateral line, eyes, mouth details, etc. The symbols aren’t standardised since it’s an aide to our memory for identification after returning to shore, rather than a reference for others, so we might use lines or shapes for fins.

FLEMMS diagram of a haddock (Melanogrammus aeglefinus)
FLEMMS diagram of a haddock (Melanogrammus aeglefinus)

This fish has three dorsal (the first prominent) and two ventral fins which, in UK waters, shows that it is a member of the cod family. There is a curved lateral line, the upper jaw extends below the lower jaw, which has a small barbel. There is a black marking just below the lateral line. The combination of the first dorsal fin and the black ‘thumbprint’ shows that this is a Haddock (Melanogrammus aeglefinus).

FLEMMS diagram of a Shanny (Lipophyrys pholis)
FLEMMS diagram of a Shanny (Lipophyrys pholis)

This is a fish with a long single dorsal fin and a single ventral fin. The tail is convex. The head is complex, with prominent bulging eyes, and thick lips with the upper lips horizontal and slightly protruding over the lower lips. The fish is around hand size with blotchy markings. The single dorsal fin and the lack of head tentacles identifies this as a Shanny (Lipophyrys pholis).

After lunch, we went to the Blue Reef Aquarium to practise on more mobile and less cooperative fish, which included blennies, gobies, wrasse, and skates and rays.

Tompot Blenny (Parablennius gattorugine)
Tompot Blenny (Parablennius gattorugine)
CC BY-SA Duncan Greenhill

We returned to the lab at Cornwall College and had another brief presentation on some of the difficulties and confusions we might face trying to identify fish such as fish keeping fins folded down (which causes us to miscount), and changes in colouration and pattern as the fish matures or changes sex. There was a perfect end to the day with a course meal looking out over the clifftop across Great Western Beach as the surf rolled in and we wondered about conditions for the following morning.

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Protected seas? It’s a long story

One thing you realise when you start looking at marine protected areas in the UK is that it’s complicated. Various acronyms crop up and are sometimes (and incorrectly) used interchangeably, and working out who’s responsible for which patch of sea can be difficult. So what types of MPAs are there in the UK, who is responsible for them, and what bits of legislation make them possible and protect them?

Dyfi Estuary. Photo credit: Nigel Callaghan
Dyfi Estuary at low tide, looking east.
Photo credit: copyright Nigel Callaghan, CC BY SA.

There are five elements to MPAs in the UK. These are SACs, SPAs, SSSIs, MCZs and RAMSAR sites. SACs [1, 2] are Special Areas of Conservation, and SPAs [3, 4] are Special Protection Areas. SACs and SPAs have their origins in the Berne Convention, which came into force in 1982 and covers the conservation of natural habitats and endangered species in Europe. The convention also covers migratory species so some countries in Africa and South America have also signed the convention. Ten years later the European Union passed two directives to implement the convention: the habitats directive, which gives rise to SACs, and the birds directive, which gives rise to SPAs.

The main aim of SACs is to protect habitats. Which habitats? Well, any habitats listed in annex I and those with any species listed in annex II mean that a SAC will need to be designated. What this means is that even within the area of an SAC, the majority of species will not be explicitly protected. However, other protection is normally also used, and SACs (and SPAs) are usually given SSSI status when they are created as well. The main aim of SPAs is to protect birds and their habitats, and again this applies to particular species listed in an annex. Taken together, SACs and SPAs form a network of protected areas across Europe called the Natura 2000 [5, 6] network. Natura 2000 sites that have a marine component are sometimes called European Marine Sites.

SSSIs are Special Sites of Scientific Interest, the majority of which are on land. Some SSSIs cover intertidal areas and some include areas that are permanently covered by seawater. They have a long history, with the first being created from legislation passed in 1949 [7]. The main piece of modern legislation that protects them is the Wildlife and Countryside Act 1981 [8, 9], with further protection being provided through the Countryside and Rights of Way Act 2000. SSSIs are designated by different organisations in different areas of the UK. These are Natural England, Natural Resources Wales (which was previously the Countryside Council for Wales until April 2013), Scottish Natural Heritage, and the DoENI (Department of the Environment Northern Ireland). SSSIs are the basis of much of the other forms of protection in the UK and most other designations are based around existing SSSIs. The sites are inspected every seven years.

MCZs are a relatively new form of protected area, and were made possible by a range of legislation. Each area of the UK has responsibility for its own territorial waters out to 12 miles from the coast. The Marine and Coastal Access Act 2009 [10, 11] covered the English and Welsh territorial waters, and UK offshore areas (out to the limits of the continental shelf). An exception to this is Scotland, which passed its own marine act in 2010 [12, 13], and retains responsibility for both territorial and offshore waters in its area. Confusingly, what would be an MCZ in any other area of the UK is called an MPA in Scotland. Northern Ireland passed its marine act in 2013 [14] and is responsible for its own territorial waters.

RAMSAR sites, like SSSIs, also have a long history. The RAMSAR convention is an international treaty created in 1971 to protect wetland sites of international importance. The first UK RAMSAR sites were created in 1976 [15].

At first glance, it seems that the seas around the UK are well protected. As we’ll see in later posts, that’s not quite the whole story.


1SACs with a marine component (JNCC)
2SACs (Natural England)
3SPAs with a marine component (JNCC)
4SPAs (Natural England)
5Natura 2000 (EU Commission)
6Natura 2000 (Natural England)
7NE306 Sites of Special Scientific Interest (Natural England)
8Wildlife and Countryside Act 1981 (JNCC)
9Wildlife and Countryside Act 1981 (Wikipedia)
10Marine and Coastal Access Act 2009 (JNCC)
11Marine and Coastal Access Act 2009 (Wikipedia)
12Marine (Scotland) Act 2010 (Scottish Government)
13Marine (Scotland) Act 2010 (Wikipedia)
14Marine Act Northern Ireland (DoENI)
15RAMSAR sites (JNCC)

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Lundy – An island of firsts

As this is the first post, I thought I’d look at another first of the marine enviroment, at least here in the UK: Lundy island. It was the first Marine Nature Reserve (MNR), the site of the first no-take zone, and the first Marine Conservation Zone (MCZ).

The island sits in the Bristol Channel about 19 kilometres off the Devon coast. It’s around five kilometres long and just over one kilometre wide, and runs roughly north-south. The importance of the island’s natural history, both above and below the water, has been recognised for a long time. The seas around the island became a voluntary marine nature reserve in 1971 and the entire island itself was listed as a Site of Special Scientific Interest (SSSI) in 1976.

Grey Seal. Photo credit: Andreas Trepte
Grey Seal. Photo credit: Andreas Trepte

The voluntary MNR was, as it name suggests, voluntary, but as time has progressed, the protection for Lundy has increased. A key point came in 1981 when the Wildlife and Countryside Act became law because it allowed marine nature reserves to be established with legal protection in much the same way that nature reserves on land could be protected. The marine nature reserve gained its statutory (legal) protection in 1986.

Lundy is also unusual in having a no-take zone off the eastern coast of the island where all forms of fishing are banned. This protects habitats and animals (such as the pink sea fan) from damage by fishing gear, and allows populations to build up and then spill out into surrounding areas. The NTZ was set up in 2003 and is legally enforceable under a fisheries bye-law, and a survey programme was undertaken to monitor the effects. By 2007, the number of lobsters above the minimum size that could be landed had increased by over 400%[1], despite the fact that the NTZ is relatively small at around 3.3 km2[2] compared to over 30 square kilometres covered by the MNR).

The area got another layer of protection in 2005 when the area covered by the MNR was designated as a Special Area of Conservation (SAC). SACs are designed to protect certain habitats and particular species, and in the case of Lundy the habitats selected for protection were most importantly the reefs, and to a lesser extent the submerged and partially submerged sea caves, and sandbanks that are partially covered by seawater[3]. Lundy is a breeding site for the grey seal (Halichoerus grypus), the largest sea in UK waters and globally rare, and some seals pup in sea caves in the intertidal zone.

Finally, in January 2010, the seas surrounding Lundy (and other existing MNRs such as Skomer) became Marine Conservation Zones (MCZs), and are meant to be the first of a network of MCZs in the UK, although controversially, only 27 of 127 potential sites received approval in the first round of designations[4].

Lundy is an island geologically as well as literally. It’s not only an island of land but also an island of hard surfaces in a sea of surrounding soft sediments. It’s mostly composed of granite, with some slate at the southern end and below the water the reefs extend offshore (for a kilometre on the western side) before dropping to relatively deep water (30 to 40 metres) and softer sediments. The tidal currents in and out of the Bristol Channel are strong and the tidal range can be up to nine metres. The tidal regime and the complex rocky environment gives rise to a mosaic of different habitats. The underwater cliffs and overhangs are home to a variety of marine invertebrates, including all five British species of cup corals[5]. Filter feeders like anemones, corals, bryzoans and sea squirts thrive here. The seas off Lundy island are outstanding in conservation terms and the island fully deserves its list of firsts.



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