Archive for the 'Connectivity' Category

Microplastic pollution – a serious threat to marine ecosystems

Text and photos copyright (c) 2013 Erkki “Eric” Siirila, all rights reserved

Pioneering research has shown that plastic waste entering the ocean may have more serious negative effects on marine life than what was previously thought. Two studies published in Current Biology concentrate on the ecosystem effects of microplastic fragments less than 1 mm in diameter. The very small pieces of plastic have been polluting the ocean for about half a century.

Previous research has concentrated on the effects of bigger plastic objects in the marine ecosystem. This time the focus is on the fragments, which are produced for example as a result of gradual breakdown of plastic bottles in nature.

The tiny plastic particles are so small that wastewater treatment plants cannot stop them from entering the sea. A serious challenge for waste management is that this pollution does not originate only in what we normally consider plastic. The sources include synthetic textiles e.g. polyester – many of our clothes release a high number of microscopic pieces of plastic fibre when they are washed. Microbeads from cosmetic facial scrubs are one more source of harmful plastic particles. On the shores and in the sea, the microscopic plastic waste sinks into the sediments in high concentrations.

An additional problem with microplastics is that, in addition to the direct effects, they transfer harmful chemicals to marine organisms eating them. This was shown to take place in the case of lugworms by Mark Browne and his colleagues (link to Abstract). Lugworms (Wikipedia Lugworm) are an example of a common North Atlantic species using the sediments as food source. Starfish and sea cucumbers have similar feeding strategies. Mark Browne’s work was completed at Plymouth University, UK.

Plastic waste entering the Atlantic via Rio de la Plata (River Plate), Buenos Aires, Argentina.

Plastic waste entering the Atlantic via Rio de la Plata (River Plate), Buenos Aires, Argentina.

The harmful substances within the microplastics include antimicrobials, hydrocarbons and flame retardants, which are often persistent and may reduce health and biodiversity. Furthermore, minute plastic particles concentrate substances from the surrounding water on their surface: to name two examples, detergents and pesticides can be detected. The chemicals may be carried over to the next predators in the food chain – lugworms are eaten by flounders and wading birds. The harmful substances could also accumulate in the top predators, perhaps even in us humans. If lugworms are seriously affected, as they are, the whole food chain could be subject to significant adverse effects.

In the study by Stephanie Wright, University of Exeter, UK, and her colleagues, it was found that those lugworms which (in laboratory tanks) were subject to varying levels of plastic contamination, gained less weight than the worms in a clean environment. Consequently, the worms suffering from the consequences of plastic pollution had less energy for growth and reproduction. The worms were also likely to be less efficient in their important ecosystem service, i.e. in eating and keeping the sediments healthy and oxygenated for other animals. The article by Wright et al. is here: http://download.cell.com/current-biology/pdf/PIIS0960982213013432.pdf?intermediate=true .

When interviewed by the BBC, Dr Browne summarised his earlier findings relating to 18 sediment samples from the beaches in several countries: “We found that there was no sample from around the world that did not contain pieces of microplastic.”

Based on these two ground-breaking articles in Current Biology, there seems to be an urgent need to develop the use practices and waste management techniques of plastic products in our societies. This is an important coastal and marine conservation issue.

In addition to the material published in Current Biology, summaries published by the British BBC and The Guardian, were helpful in the preparation of this Coastal Challenges’ article.

 

Connectivity, coral reefs and marine parks

Water is in constant motion and transports sediments, nutrients and pollutants. At least during one life stage, most marine organisms move within the water stream, either passively or actively. Connectivity is the word used to describe all these movements.

Connectivity is an important consideration in coastal management and in the design of marine protected areas (MPAs) and MPA networks. When fish larvae and fertilized coral eggs move in water currents from one place to another, these movements become crucial for the location of the new generation of these animals.

Most marine organisms on reefs and in coastal waters are relatively sessile during most of their life. This sedentary lifestyle is abandoned during reproduction: most reef species produce pelagic eggs which become pelagic larvae. Some of these pelagic larvae become fish. When fish grow older, they may travel to another location, while juvenile coral colonies will generate a reef where the coral eggs and larvae end up – in case the marine environment is suitable for reef growth.

Some habitats are critical to the early developmental stages of fish, lobster, and shrimp, while others serve as spawning or feeding grounds. Marine organisms also migrate daily and/or seasonally between habitats. The daily shifts commonly involve nightly feeding migrations between feeding and resting habitat. In some fish species, these daily movements lead to nutrient transfer between seagrass/mangrove areas and the coral reef.

Connectivity is an important consideration in the management of this Red Sea coral reef surrounding an Egyptian island. Photo (c) 2010 Erkki Siirila.

The marine ecosystem is so complex that many connectivity issues are poorly known. Nevertheless, this field of marine ecology is advancing: better understanding is crucial for sound marine management.  An example of these advances is the publication in 2010 of “Preserving Reef Connectivity: A Handbook for Marine Protected Area Managers”, which can be found here: Handbook

Special attention in the Handbook (written by P.F. Sale et al., edited by Lisa Benedetti and published by UNU-INWEH), which is the main source for this Coastal Challenges’ article, is given to populational connectivity. This includes

  1. Evolutionary (genetic) connectivity; and
  2. Demographic (ecological) connectivity.

In the Handbook, number 1 is said to “be informative when considering long-term (evolutionary) and large-scale biogeographic dispersal patterns of organisms. It can also be useful for managers wanting to assess the genetic uniqueness of populations when making decisions concerning biodiversity preservation.”

Number 2 “involves the extent of linkages that occurs among nearby local populations of a species due to the exchange of individuals”. This type of connectivity is important for the design and management of marine protected areas (MPAs) and no-take fishery reserves (NTRs), and when we want to know the ideal amount of coral reef habitat to protect.

The results of recent investigations are clear: pelagic larvae do not drift aimlessly in the ocean. They use for example sensory capabilities to minimize the extent of dispersal. In many species the larvae have the capability to settle on suitable reef habitat and specific microhabitats.

Connectivity amongst populations of reef species is primarily due to dispersal during larval life; demographic connectivity takes place on scales of up to tens of kilometers. The concept of demographically well connected populations for example across the Caribbean is not true and belongs to the past. Only genetic (evolutionary) connectivity links these habitats far away from each other, when larvae occasionally get transported beyond the usual dispersal range.

When marine parks are intended to function as fisheries management tools, the smaller scale of demographic connectivity should be taken into consideration in the MPA design – and in the design of MPA networks. This type of connectivity is worth remembering also when coral reefs experience massive destruction (hurricanes, bleaching, crown-of-thorns attacks): demographic connectivity defines the limits of natural re-seeding.

Coastal development may damage important inshore areas used by developing fishes and other organisms. For example, pathways between these and offshore habitats may be disrupted. Negative impacts during an organism’s early life stages may also have consequences for the abundance of adults. In addition, linked food webs may be affected. Furthermore, daily or seasonal migration routes could be disrupted. – MPAs and MPA networks should be large enough to encompass the interlinked habitats.

Spawning aggregations of groupers are an exciting phenomenon in many parts of tropical seas. During large scale oceanic movements and gatherings, species behaving in this manner are vulnerable to overfishing. The spawning sites should be part of no-take fishery reserves (NTRs). (More information on NTRs specifically can be found in “Fully protected marine reserves: a guide” by Callum M. Roberts and Julie P. Hawkins, 2000, which can be downloaded from here: Guide)

In general, NTRs promote fish survival and reproduction even when serious overfishing takes place in the surrounding area. Studies have shown that four positive changes inside NTRs take place. These changes, which may benefit fished populations outside reserves, are summarized in the Handbook on reef connectivity. They are:

  1. Increased reproductive output (increases in fish abundance, spawning biomass, mean age, and body size result in this change)
  2. Higher net export of juveniles and adults (to surrounding fished areas);
  3. Higher net export of eggs and larvae (to surrounding fished areas); and
  4. Protection and recovery of crucial habitats/ecosystems (key underwater areas for the fished species)

The NTR studies indicate that when neighboring NTRs are not more than 10-30 km apart, appropriate levels of populational connectivity exist for most reef species targeted by fishermen. As indicated above, early hydrodynamic models predicted dispersal distances of hundreds of kilometers. Based on new evidence, even relatively small MPAs may be self-sustaining.

In a world of climate change, it is important that coral reefs and other coastal ecosystems are managed as effectively as possible. Their natural resilience can be supported by taking connectivity into consideration.