10 Marine Biodiversity: Spatial Patterns

Dr. Felix Bast

1. Learning outcomes

1.1. To learn the overview and importance of marine biodiversity

1.2. To learn about various ecosystem goods and services provided by the marine biodiversity

1.3. To learn about zonal classification of marine biodiversity

1.4. To learn about global patterns of marine biodiversity

1.5. To learn about threats and challenges that are being faced by marine biodiversity

2.    Concept map

3.      Description

3.1. Introduction

World’s oceans encompasses vast majority of the planet’s surface, more than seventy percentage, yet largely remain unexplored. The Marine ecosystem is by far the largest ecosystem on this planet harbouring rich diversity of living organisms which play a key role in the sustenance of life on this planet. As per the CBD, biological diversity, or biodiversity, is defined as “The variability among living organisms from all sources including inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species and of ecosystems.” The life is believed to have originated in the ocean around 3.7 billion years ago, and much of the organic evolution happened there; it was only around 400 million years ago that the first terrestrial colonization of Marine plants, and subsequently animals, begun. Although the Marine biodiversity is playing a number of crucial roles for the sustenance of terrestrial organisms including us, its biodiversity largely remain inconspicuous. For example, it is estimated that more than 65% of the planet’s oxygen production happen in ocean, by means of two microalgal species namely Prochlorococcus and Synechococcus, and without these two species alone, the rest of the aerobic life on this planet would be asphyxiated. These and other marine plants fix vast majority of carbon dioxide from the atmosphere at the primary tropic level of the oceanic ecosystem. Carbon fixation by the Marine algae acts as the natural atmospheric carbon capture mechanism, through which level of CO2 in the atmosphere had been in control for billions of years. When these surface algae, called phytoplankton, and other marine organisms die, it sinks to the ocean bed carrying the fixed carbon, which later compressed into what we refer these days as fossil fuel. Although world’s major oceans are contiguous to encompass one giant system, biodiversity of which do not have such a uniform pattern. There are distinct biologically mediated habitats including seaweed beds, seagrass meadows and coral reef ecosystems, which has a high degree of both species richness and species evenness.

3.2. Ecosystem goods and services provided by the Marine biodiversity

At the core of the topic of marine biodiversity is an understanding of why such a biodiversity matters to us. Marine biodiversity consists of all kinds of Marine organisms, including a cellular viruses, prokaryotes (bacteria, blue-green algae) and eukaryotes (protists, algae, plants, fungi and animals). The term Ecosystem goods and services refers how an ecosystem is beneficial to the humanity by means of tangible goods and intangible services that it provides us. Marine biodiversity provides a number of goods, the products that are beneficial to the humanity. Marine life-based products include fisheries and other food resources including edible seaweeds, seaweed based industrial products such as alginate, agar, Carrageenan etc., pearl oysters, pharmaceutical, nutraceutical and biochemical products from the Marine organisms, other industrial products from the Marine organisms including paints, fertilizer, fodder, cosmetics, Antifouling substances, glues and so on. There are a number of regulating services that the Marine biodiversity provides; this include global oxygen production through phytoplankton photosynthesis, atmospheric natural carbon capture and sequestration by marine algae, nutrient recycling via a number of biogeochemical cycles. The Marine biodiversity also provides cultural services that are non-material, so as other supporting services that offer no direct benefit to human beings. For example, marine Aprodite spines recently discovered to have potential applications in photon engineering, communication and Biomedical engineering. Marine microbes have been recently discovered to be producing electricity that has potential applications in providing economically viable electricity in rural areas. Marine bivalve shells have been described to have potentials for developing into tougher water resistance ceramics for biomedical and structural engineering applications. Other intangible services that the Marine biodiversity offers include the so-called ‘warm glow’ services, including bequest value and existence value. Bequest value refers the public perception that the Marine biodiversity is extremely important for our generations of posterity even though some of the organisms might not yet be useful to us directly; its utility remain to be discovered. Existence value refers the value accredited to the Marine ecosystem just by knowing its existence. Popular media such as BBC’s Blue Planet documentary series have had a tremendous impact on the people’s perception of Marine biodiversity such that those who have never saw a coral reef ecosystem by themselves could still be appreciative of its importance by its mere existence. It is estimated that vast majority of Marine biodiversity is remain to be discovered; some say humanity knows more about moon than our own oceans. Marine ecosystem goods and services, therefore, also includes future unknown and speculative benefits to the humanity.

3.3. Intertidal biodiversity

Arguably, the marine habitat with richest biodiversity in offing is indeed intertidal region. Intertidal region, also called eulittoral region, is the coastal area between high tide mark and low tide mark, therefore, this area periodically get submerged in water during high tides, while get exposed during low tides (Fig. 1).

Figure 1. Various zones, habitats and topological features of marine ecosystem.

Above the high tide mark where the surf gets splashed is called splash zone (also called supralittoral zone), which is also included in the intertidal region. A vast majority of described seaweed species are from intertidal region. This region also support a wide variety of marine animals as well, including crabs, sea cucumber, barnacles, limpets, isopods, mussels, starfish, sea anemones and so on. Tidal range, the distance between high water mark and low water mark, defines the intertidal region. The tidal range is influenced by a number of factors including topography of coast, gravitational force of moon and so on. There is a substantial variation in the tidal range spatially and temporally; at some locations such as that in Canada and United Kingdom, tidal range could be as high as 15 meters, while most of the other coastal regions have average tidal range of about 2 meters. The tidal range is high during spring tides and low during near tides; therefore, researchers who work with intertidal biodiversity typically plan the expedition during spring tides to maximize their sample collection.

3.4. Neritic zone biodiversity

Continental shelf is the region beyond intertidal habitat towards Open Ocean where the sea is moderately shallow, before making a steep incline at the fringe of the shelf. Continental shelf ecosystem is also known as the neritic zone. The region immediately below the low water mark is called sublittoral zone, which the region of neritic zone nearest to the coast. Geographically, the continental shelf is part of the continental crust. Typical depth of this region is around 200 meters. This region is perhaps the most exploited marine habitat by the humanity for the fishery resources. The part of continental shelf that lies within 22.5 km from the coast is known as territorial waters, a region that is considered to be part of the national jurisdiction of respective country, while the region including territorial waters and extending up to 280 km into the sea is called exclusive economic zone. Due to the shallow nature, the neritic zone largely encompass photic zone, where sunlight can penetrate and thereby supports primary productivity. In contrast, the abyssal plain of deep ocean, where the depth is between 3000 meters and 6000 meters, remains mostly a barren desert for the life.

3.5. Pelagic zone biodiversity

Pelagic zone is the water column of the open ocean beyond continental shelf; the continental shelf makes a steep incline known as continental slope towards the open ocean. The pelagic zone, with an average depth of around 2 kilometers, can be further divided into various sub-zones depending on the depth. The term pelagic typically refers the surface waters of the ocean where abundant biodiversity is present. The biodiversity consists of planktonic organisms that float on the sea surface, which include phytoplanktons (planktonic plants) and zooplanktons (planktonic invertebrate animals). Phytoplanktons are the largest primary producers in the world; as stated earlier, these tiny organisms fix more than 65% of the CO2 from the atmosphere and generate vast majority of the oxygen that we breath. Phytoplankton encompass both eukaryotic microalgae, as well as prokaryotic cyanobacteria (blue-green algae). Cyanobacterial phytoplankton, owing to its small size, is often referred to as picoplankton. The term femtoplankton refers to marine viruses. Planktons, by definition, cannot swim actively against the current; those that can swim against the current are termed nektons. Nektonic biodiversity of pelagic zone largely consists of bony or cartilaginous fishes, mollusks such as scallops and squids, and crustaceans (arthropods) such as lobsters and crabs.

3.6. Benthic biodiversity

In contrast, benthic zone refers to the region at the very bottom of the open ocean, the sea ed (the abyssal plain). The region just above the benthic zone is called demersal zone. Both benthic and demersal zones are extremely less biodiverse region of the open ocean (however, benthic region of coastal regions have very high biodiversity). Organisms like bivalves, sponges, polychaete worms, snails, cephalopods, crustaceans, clams, crabs etc. that live on the benthic zone is known as benthos. Benthos are mostly filter-feeders, detrivores or saprophytes. Most of the benthic biodiversity remain unknown to the humanity, as deep-diving equipment including manned submarines and unmanned remotely operated diving vehicles, are a necessary for its discovery and retrieval, and as of this writing only very few countries have such technological capabilities. Even the pelagic marine biodiversity largely remain undiscovered. For example, it was only recently that the colossal squid (Mesonychoteuthis hamiltoni) that can grow up to 14 meters in length, had been fully described.

3.7. Hydrothermal vent system

Although benthic biodiversity thought to be minimal and largely remains unexplored, certain regions harbors unique biodiversity; one such noteworthy ecosystem is hydrothermal vent system. Submarine hydrothermal vents often forms features known as black smokers, and consists of various chemoautotrophic and thermophilic bacteria. Hydrothermal vents are found along the edge of tectonic plates, and therefore, volcanically active locations. Several deep-diving submarine missions have discovered considerable biodiversity at a number of black smokers, including one at the challenger deep, the deepest point in ocean approximately 11 kilometers below the surface in Pacific Ocean. The biodiversity other than thermophilic bacteria encompass giant tube worms, limpets, clams and shrimp that depend upon the chemoautotrophic bacteria for their sustenance, a truly remarkable biodiversity found no where else on the planet. While the average seawater temperature at the seabed is around 2C, temperature at these black smokers could reach as high as 465C, owing to the extreme pressure of the water column. One of the prevailing theories explaining the origin of life on planet earth propounds that the life originated in these submarine hydrothermal vents.

3.8. Biologically mediated marine habitats and its biodiversity

A number of highly biodiverse hotspots in the ocean are biologically-mediated habitats, habitats in part constituted by the living organisms. Prominent biologically mediated habitats of the world’s oceans include seaweed beds, seagrass meadows and coral reef ecosystem. These habitats serves as a breeding nursery ground for fishes and other marine animals, as a shelter for their sustenance, and as a refuge from predators. Seaweed beds, by far, is the most important amongst these biologically mediated habitats, as they form vast expanses of productive clonal forests supporting the ecological niche. Extensive submarine forests of giant kelp, the brown seaweed Macrocystis pyrifera have been reported from cooler parts of many shallow ocean beds of the world. Seagrass meadows also serve as a productive biologically mediated ecosystem. Genera Zostera, Phyllospadix and Halophila form predominant seagrass meadows in the world. Seaweed meadow ecosystem is extremely important and support a wide variety of biodiversity for multiple reasons; as it slows down the oceanic current, it increases sedimentation, and its extensive root system and rhizomes serves as a scaffold for reinforcing the seabed, ultimately helping to minimize coastal erosion. Both seaweed beds and seagrass meadows are amongst the most productive ecosystems of the ocean; it fixes CO2 into food to support higher trophic levels, and produces oxygen supporting the aerobic life. Some estimates suggest that seagrass meadows alone constitute 10% of the total oceanic carbon production, demonstrating the extraordinary ecosystem services that these ecosystems offer. Coral reef ecosystems, although an object of media glory, is comparatively less extensive although highly productive ecosystem, sometimes referred as ‘rainforests of the sea’. Coral reefs, albeit occupying less than 0.1% of the total oceanic area, support the life of more than 25% of the marine life, demonstrating its importance in marine biodiversity. The reef system is in fact symbiosis between an animal (Cnidarian, related to sea anemones and jellyfish) and an algae (dinoflagellate, Symbiodinium, also called “zooxanthellae”). The algae provide nutrients for the cnidarian, which grows and exudate Calcium Carbonate to support its exoskeletons, thereby bolstering the hard coral reef system. Barrier and Fringe coral reefs are most abundant, while circular atoll coral reef system are common around islands, including Laccadives. World’s largest coral reef ecosystem is found off Queensland in Australia, the Great Barrier Reef. The reef ecosystem support turf and coralline algae, multitude of tropical fish species, invertibrates including sponges, sea urchins and sea slugs, sea snakes and so on.

3.9. Global spatial patterns of marine biodiversity

Perhaps the most prominent spatial pattern of global marine biodiversity is that the most biodiverse region is off Indo-Malay archipelago, and from this place on all directions the biodiversity gradually decreases. One explanation for this apparent bizarre phenomenon is that the Indonesian archipelago provides a richly diverse habitats that can support various living forms with extremely high phylogenetic diversity (encompassing highly evolutionarily distant species) such that the system has unusually high degree of ecological resilience (the ability to spring back to normal after major natural catastrophes). Various studies have confirmed this longitudinal pattern of global marine biodiversity. Such a clear pattern for latitudinal gradient of marine biodiversity is nonexistent. In case of terrestrial biodiversity, the biodiversity decreases progressively polewards (i.e., from equator to either south pole or north pole, the biodiversity decreases). A similar trend of decrease in biodiversity from equator to north pole had ben document in case of marine biodiversity. However, such an obvious pattern in nonexistent in the case of southern hemisphere; ocean in extreme southern latitudes off Antarctica is highly biodiverse and the diversity is comparable that of the tropics. Another obvious spatial pattern is that pelagic marine biodiversity is higher at shallow coastal waters than at deep open ocean waters, as the coasts support a wide range of habitats for the multitude of life forms. One estimate conclude that while there are only 1200 species of pelagic fish, there are around 13000 species of coastal fish. However, the benthic biodiversity of soft sedimentary seabed tend to increase from shallow coastal region to deep sea. Another pattern is that the marine biodiversity tend to magnify in benthic zone rather than pelagic zone, the reason is thought to be due to the origination of marine fauna at the sediments of benthic region rather than in pelagic water column.

3.10.   Threats to marine biodiversity

There are a number of anthropogenic threats to the global marine biodiversity. Perhaps the most serious threat are the effects of climate change. There are three major effects of climate change that affects marine biodiversity; first and foremost amongst them, albeit inconspicuous, is the phenomenon of ocean acidification. With rising atmospheric CO2 level, a substantial portion of it get mixed up with seawater and converted into carbonic acid. This result in the acidification of global oceans. Since the industrial revolution of 18th century, the average pH of world’s oceans have decreased from 8.19 to 8.07. This change might look a very slight at first, but remember that pH is expressed in logarithmic scale, and a drop of 0.1 pH unit indicate a net acidification of 28.8 %. The consequences of ocean acidification is widespread and alarming. Unicellular algae are especially vulnerable to the acidification; algal symbionts-zooxanthellae- of corals dies, killing the whole coral reef ecosystem. A vast majority of coral reef ecosystems in the world have been threatened by this phenomenon, with a large number of reef systems already collapsed- a phenomenon called coral bleaching, the irrevocable death of coral reefs. Rising ocean acidity also affects phytoplanktons-the largest primary producers of the world. Another effect of climate change is rising seawater temperature. Rising water temperature has a direct effect on the survival and life history of organisms. It also lead to changes in species distribution patterns and species dispersal from warm tropics to warming polar regions. Rising seawater temperature also affects algae and algal symbionts of corals; combined with ocean acidification, this double whammy is truly disastrous. The third consequence of climate change is rising seawater levels. Although this might not have direct implication to marine biodiversity, the seawater level rise would lead to the collapse of many fringe ecosystems, including estuaries, wetland and mangrove ecosystems, with associated loss of biodiversity. Many of the marine organisms, including a number of fish species, migrate to estuaries and riverine habitats for spawning.

Over exploration of marine resources are a major threat to marine biodiversity, and this problem is especially prone to the coastal regions. Fish resources of the world is rapidly diminishing due to over exploitation and trawler fishing that indiscriminately catches bottom-dwelling key species as well as the juvenile forms of all fishes. Trawling also catches mollusks and echinoderms, adversely affecting its biodiversity. A form of fishing popular in many countries including Philippines is dynamite fishing where a major submarine explosion kills vast number of fish altogether. However, this also decimate the entire habitat with all other life forms. Fishing using cyanide to catch the live fishes also have huge repercussions to the marine biodiversity. Other major threats to marine biodiversity include anthropogenic pollution. Major pollutants that affect marine biodiversity are PCBs, Dioxins and Heavy Metals, most of which get progressively bio accumulated to higher trophic levels. Higher nutrient load also results in algal blooms leading to phenomenon of eutrophication. This causes hypoxia, reduced dissolved oxygen concentration, leading to the collapse of the entire ecosystem. It is estimated that 67% of world’s population lives along the coast, with in 60 km from the coast, and a number of major cities of the world are located at the coast. This demographic pattern exerts tremendous stress to the marine biodiversity. Marine litter is a major problem in world’s oceans; there are a number of massive litter islands reported in open sea around the world. Littering by microplastics is revealing itself to be a major issue, as this could get into the bodies of a number of marine organisms. Habitat loss is yet another major issue. Major civil construction projects, including dams, port and reclamations of estuaries have implications on the marine biodiversity. Marine species invasion through ballast water has altered a number of natural environments. Examples include introduction of invasive seaweed, the ‘killer algae’ Caulerpa taxifolia into Mediterranean Sea, and the ctenophore Mnemiopsis leidgi from US East Coast to Black Sea.

4.      Summary

4.1. Marine biodiversity, despite playing crucial importance in global biogeochemical cycles and nutrient fluxes, remain poorly studied.

4.2. Marine Biodiversity is traditional classified according to various zones and depth profiles, including neritic, limnetic, photic, intertidal, subtidal and benthic diversity

4.3. There are a number of biologically mediated habitats that host an extraordinary diversity of marine flora and fauna; these are seaweed beds, seagrass meadows and coral reef system. Apart from these, hydrothermal vents also has high biodiversity.

4.4. A clear longitudinal gradation in marine biodiversity is that from Indonesian archipelago in all directions, the biodiversity gradually decreases.

4.5. Latitudinal gradation is less apparent. While diversity decreases with increasing latitudes in Northern hemisphere, same is not true in Southern Hemisphere where tropics as well as Antarctic Ocean have comparable diversity

4.6. In pelagic zone, coastal waters have more biodiversity than open ocean waters. However in case of benthic zone, biodiversity increases from coast to open sea, although this pattern is less obvious

4.7. Major threats to marine biodiversity include climate change, pollution, over exploitation, habitat loss, and species invasion.