What is happening to our sea?

© GENERALITAT DE CATALUNYA

The Mediterranean Sea is a hotspot of biodiversity, hosting 4% to 18% of the world's known marine species

But it is also a hotspot of Climate Change, which puts at risk this biodiversity. Rising water temperatures cause rapid changes in this rich biodiversity due to changes in the composition and abundance of species.

Its temperature is increasing

Today, the average annual temperature in the Mediterranean basin is 1.5 °C higher than during the pre-industrial era (1880-1899) and heat waves (periods of very hot weather) are likely to be more frequent and/or more extreme. 

Projected temperature increases by 2100 vary between 1.8°C and 3.5°C on average compared to the period between 1961 and 1990. The Balearic Islands, the north-western Ionian Sea, the Aegean Sea and the eastern Mediterranean basin have been identified as the areas with the maximum increase in sea surface temperatures. Mediterranean sea surface warming is currently estimated to be 0.4°C per decade between 1985 and 2006 (0.3°C per decade in the western basin and 0.5°C per decade in the eastern basin).

The sea level is rising

The Mediterranean Sea level has risen between 1945 and 2000 at a rate of 0.7 mm per year and between 1970 and 2006 at the level of 1.1 mm per year. 

There has been a sharp increase during the last two decades as sea level rise reached about 3 mm per year. Circulation patterns are also altered, generating changes in sea level. In southern Italy, substantial coastal inundation is expected by 2100. Significant shoreline modifications are also expected in other areas, like the Balearic Islands. The accelerating ice loss in Greenland and Antarctic ice sheets implies a significant risk for additional sea-level rise in the Mediterranean basin. Many coastal regions will experience increased inundations,  storm flooding and accelerated coastal erosion.

Seawater PH is decreasing

CO uptake by the Mediterranean is expected to lead, by 2100, to acidification of 0.018 to 0.028 pH units per decade.

Seawater pH has decreased by 0.1 pH units since the preindustrial period, that corresponds to an increase in oceanic acidity of about 30% and in recent decades, subjecting marine organisms to environmental stress. 

Southernisation and tropicalisation

In general, cold-water species become less abundant or become locally extinct, and warm-water species become more abundant and widespread, a process known as “southernisation“. 

Cold-water species seek colder environments and change their geographical distribution by heading towards the poles, but these movements are physically constrained, as in the Gulf of Lion, the northern Adriatic and the northern Aegean, which are the coldest sectors of the Mediterranean. 

The successful geographic spread of warm-water species is the most obvious phenomenon related to global warming. Two major processes of change, involving both native and non-native species, describe the increasing importance of thermophilic biota in the Mediterranean Sea. The first process is often referred to as “southernisation”, that is, southern species that move north; for the second process, we use the term “tropicalisation”, which refers to exotic species of tropical origin (often from the Suez Canal) that spread across the Mediterranean.

These two processes of change contribute to the progressive restructuring of the composition of Mediterranean ecosystem communities, with serious ecological impacts caused by tropical invaders.

© Cristina Linares

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Mass mortality

Increased water temperatures also lead to mass mortality events (MMEs) of about 100 species belonging to various groups of organisms such as corals, sponges or molluscs. The most impressive phenomenon happened in 1999, when a large-scale and unprecedented MME affected populations on the French and Italian coasts. Other large-scale MMEs have been reported since this event, as well as many smaller ones, which tend to be more restricted in geographic extent and/or number of species affected. Most Mediterranean regions have been affected by this phenomenon in recent decades.

Generally, considering that species affected by mass mortalities are long-lived (tens to hundreds of years), recovery is an extremely difficult and long-term process. In addition, as these populations undergo increasing marine heat waves there are more frequent reports of recurrent mortalities, which make recovery more difficult. Indeed, the long-term monitoring series available to us unfortunately indicate that local extinction processes are taking place in many cases.

© Fernando santos · Unsplash

The meadows of Posidonia oceanica are at risk

The Mediterranean meadows of Posidonia seagrass (Posidonia oceanica) are one of the most powerful natural defenses against climate change and a source of a variety of ecosystem services. Seagrass meadows are also highly resistant to microbial degradation, so carbon is not degraded when deposited on the seabed and can remain there unaltered for centuries. 

Despite this, posidonia has an upper termal tolerance limit of 28°C and increasing sea surface temperatures above this limit induce thermal stress, promoting algal growth. Depending on the water temperature, the species reproduces sexually by flowering or asexually by cloning. The increasing frequency of posidonia blooms in the Mediterranean are a sign of warmer water, but also of an adaptive advantage to survive in a future warmer ocean.

Invasions of exotic species

The Mediterranean has become the world’s hotspot for biological invasions. Some exotic species enter the Mediterranean through the Suez Canal. Others are translocated by ships, aquaculture activities or released from aquariums.

To date, nearly 1000 non-native marine animal and plant species have been recorded in the Mediterranean and almost 700 of them have established permanent populations. Many of these animals and plants have a tropical origin and they are being favoured by the warming conditions, as evidenced by their rapid geographic expansion.

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Invasive algae

© Cristina Linares. UB 

Caulerpa cylindracea is an invasive green alga, native to south-west Australia and which probably entered the Mediterranean through the ballast water of ships. It can grow extensively and develop massive kelp forests that have severe ecological impacts. 

Some species of Caulerpa are used for human consumption in countries such as Japan or Indonesia, but their use for food purposes is not recommended until specific studies are carried out.

Eating invasive species: new food resources, new culinary recipes

Some of the invasive species of the Mediterranean have already become a new food resource. Eastern Mediterranean fish markets already sell numerous tropical fish such as rabbitfish (Siganidae), Upeneidae, Nemipterus randalli, and crustaceans such as Portunus segnis, Erugosquilla massavensis

Some of these species are already found in the central areas of the Mediterranean and are increasingly abundant there. In Catalonia, specifically in the Ebro Delta, people have started eating blue crab to help eradicate it. Elsewhere, delicious recipes are being created, such as with rabbitfish. 

In Cyprus, rabbitfish is considered a delicacy, and sells for 21 euros per kilo. How to cook rabbitfish (Siganus luridus)? 

Here’s a recipe from Kastellorizo, Greece:

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1. Remove the guts as soon as possible.

2. Cut off its poisonous thorns.

3. Fry it. 

4. Serve with chips and salad.

Effects on fishing

Currently, Mediterranean fishing is mostly affected by overfishing, but climate change also plays an important role. Mediterranean fisheries are multi-species and multi-fleet; small pelagic species are the most important in terms of landings, and the dominant fleet is that of small-scale fisheries (SSF). Changes in primary production and increasing temperatures negatively affect optimal habitats for small pelagic fish. The European sprat, a small cold-water pelagic species, has already disappeared from commercial fisheries in the north-west Mediterranean and landings of sardines and anchovies have declined dramatically in recent decades

© Josep Maria Dacosta