Plankton

A Guide to Their Ecology and Monitoring for Water Quality

Iain M. Suthers,David Rissik,Anthony J. Richardson

• Publisher: CSIRO PUBLISHING
• Year: 2019
• ISBN:
  9781486308811
• Edition: 2
• Topic: Ciencias biológicas
• Subtopic: Zoología

Healthy waterways and oceans are essential for our increasingly urbanised world. Yet monitoring water quality in aquatic environments is a challenge, as it varies from hour to hour due to stormwater and currents. Being at the base of the aquatic food web and present in huge numbers, plankton are strongly influenced by changes in environment and provide an indication of water quality integrated over days and weeks. Plankton are the aquatic version of a canary in a coal mine. They are also vital for our existence, providing not only food for fish, seabirds, seals and sharks, but producing oxygen, cycling nutrients, processing pollutants, and removing carbon dioxide from our atmosphere.

This second edition of Plankton is a fully updated introduction to the biology, ecology and identification of plankton and their use in monitoring water quality. It includes expanded, illustrated descriptions of all major groups of freshwater, coastal and marine phytoplankton and zooplankton and a new chapter on teaching science using plankton. Best practice methods for plankton sampling and monitoring programs are presented using case studies, along with explanations of how to analyse and interpret sampling data.

Plankton is an invaluable reference for teachers and students, environmental managers, ecologists, estuary and catchment management committees, and coastal engineers.

1

The importance of plankton

Iain M. Suthers, Anthony J. Richardson and David Rissik

Phytoplankton and zooplankton – tiny drifting plants and animals – are vital components of aquatic systems. It is sobering to think that all of the large and charismatic animals in aquatic systems that we are familiar with – the fish, seabirds and mammals – are minor components of the food web when the biomass of different groups is considered (Fig. 1.1). Aquatic systems are dominated by very small organisms that we rarely see: bacteria, phytoplankton and zooplankton. This huge biomass of very small organisms means that most ecosystem services provided by aquatic systems are provided by plankton. Plankton not only provide the food for higher trophic levels such as fish, seabirds, penguins, seals and sharks, but produce oxygen, cycle nutrients, process many of the pollutants that humans dispose of through our waterways, and help to remove carbon dioxide from our atmosphere. Without the diverse roles of plankton, our waterways and oceans would be virtually devoid of life and our planet would be very different.

Being at the base of the food web and in such huge numbers, plankton are strongly influenced by water quality because they cannot isolate themselves as oysters do by closing their shells in adverse conditions. Plankton are effectively our aquatic ‘canaries-in-a-coal mine’, providing an indication of the effects of hourly changes in water quality integrated over days and weeks.

Management of water quality can be supported by having a broad understanding of plankton and their interaction with the environment. Phytoplankton respond rapidly within days to changes in light, nutrients, pollution or sediment load, changes in water flow or estuarine flushing, and in response to grazing by larger zooplankton. Therefore, from a manager’s perspective, the response time of plankton is comparable to changes in water quality, which contrasts with changes in the benthic community or in fish that respond over broader scales of months or many kilometres (Fig. 1.2).

The amount and type of phytoplankton present in the water can inform managers about the health of the waterways and where management actions may be required. High biomass of phytoplankton often reflects excessive nutrient inputs (eutrophication) and this can cause problems when the phytoplankton blooms die and decay, depleting oxygen levels in the water. The types of plankton present in the water are also important, because several phytoplankton species are toxic and can be harmful to humans, but not necessarily to the vectors of the toxin, such as oysters or fish. It is important to know about harmful phytoplankton species to manage causes of blooms.

1.1 What are plankton ?

Plankton may be defined as any organisms that cannot swim against a current. Most plankton can swim or adjust their position by changing buoyancy but they lack the power to swim against a persistent current. Most plankton are microscopic, which affects their swimming ability, but some zooplankton such as jellyfish can be huge: up to 2 m in diameter (Chapter 8).

Fig. 1.1. Diagram of a marine food web off south-east Australia, with the size of spheres proportional to the biomass of each group. Plankton dominate the biomass; the large species that we know well such as fish, whales, seabirds and penguins have a minuscule biomass in comparison. The biomass is estimated from a balanced ecosystem model based on available biomass data for each marine group (‘Atlantis’, Fulton et al. 2004). This diagram shows only biomass, and if presented as production of biomass over a year, would show plankton and bacteria to massively dominate the ecosystem even more than is shown by biomass alone.

Phytoplankton, such as diatoms and dinoflagellates, grow in the presence of sunlight and nutrients such as nitrogen and phosphorus. These single-celled organisms are the ‘grasses of the sea’ and are the basis of ocean productivity. Many of these ‘plants’ – but not all – are in turn grazed by zooplankton, which is dominated by small crustaceans such as copepods, shrimps and their larvae, and by smaller single-celled microzooplankton. The amount of phytoplankton in the water column reflects the influence of several environmental factors and processes. These competing processes may be summed up as ‘bottom-up’, such as those concerning nutrients and light, which drive primary production, or ‘top-down’, such as predation by copepods or other grazers.

Phytoplankton contain photosynthetically active pigments such as chlorophyll, which enable them to use energy from sunlight to convert carbon dioxide into complex organic molecules, such as sugar or protein (i.e. they are autotrophs). Chlorophyll is used as an estimate of phytoplankton biomass. The majority of chlorophyll in tropical and subtropical coastal waters is found in the very smallest of cells – the size of bacteria, (~0.001 mm or 1 µm). These very small cells have high surface-area-to-volume ratio, allowing them to out-compete larger phytoplankton cells in the race for nutrients.

Fig. 1.2. Range of possible estuarine health and water quality indicators available, illustrating the higher trophic level and intermediate integrating period of zooplankton. Phytoplankton (Phytopl.) is often quantified as the concentration of chlorophyll-a, the primary photosynthetic pigment (Chl-a).

Exceptions abound where some of these single-celled ‘plants’ do not fix their own carbon, but engulf and consume other plant cells (i.e. they are heterotrophic like an animal and have no photosynthetic pigments). Other single-celled organisms both photosynthesise (like a plant) and they eat other organisms (like an animal). If you work on phytoplankton, the distinction between plants and animals becomes blurred. Other phytoplankton have the potential to form harmful algal blooms (HABs) – producing red tides or toxic algae – but there are only a few species responsible (just a fraction of a per cent of all phytoplankton species may be harmful; see Chapter 5 and 6). Most phytoplankton are enormously beneficial, such as those used in the aquaculture industry as food for young fish and shellfish. There are distinct forms and different sizes of the major phytoplankton groups and this book will guide you through their identification.

Zooplankton refers to the small multicellular animal life, dominated by crustaceans and some types of gelatinous animals. Zooplankton includes representatives of nearly all of the 34 major groups or phyla (a phylum is a discrete evolutionary lineage) of multicellular animals alive today. Zooplankton includes the larvae of many familiar animals that spend only a portion of their life as plankton – fish, crabs, lobsters, oysters, mussels, jellyfish and starfish – and are known as meroplankton (Chapter 2). Holoplankton spend their entire life in the plankton and include copepods, ctenophores, arrow worms and salps. Some typical benthic animals, such as snails, marine worms and even tiny fish, have some holoplanktonic species with fascinating specialised body forms.

The most abundant animals on the planet are copepods (Sections 1.2.9, 8.3.1), and may comprise over 95% of zooplankton abundance and biomass (Fig. 1.1). Only occasionally will jellyfish, ctenophores or salps predominate. There are over 12 000 species of copepods (yet only 78 species of krill!), and each species of copepod develops by moulting through six larval (naupliar) stages and five juvenile (copepodite) forms until they reach the final and sixth adult stage (Fig. 2.5). Many zooplankton species have young stages that look very different to their adults, making the study of zooplankton interesting, complex and challenging. This book will guide you through this complexity and discuss the traditional and a few modern ways one can study zooplankton.

1.2 Fun facts about plankton

With most plankton being microscopic, the amazing roles of plankton are largely hidden from us. Here we present some fun facts about plankton to highlight their many diverse, yet critical roles, which go totally unnoticed.

1.2.1 Did you know that our society is based on plankton?

You might be surprised to learn that our cars run on plankton, and many parts of cars are even built from plankton! Plankton also make most products we use in everyday life, from plumbing materials to our clothes. Here’s why. Our society is based on petroleum products – our cars, planes and trains that keep us moving, the roads we drive on, and the plastics that we use to make our everyday products. Petroleum is formed by dead zooplankton and phytoplankton sinking to an ancient seafloor. Under low oxygen conditions, plankton is not broken down by bacteria, but is buried by sediment. Over time, pressure and heat convert the plankton and sediment into sedimentary rock. If there is sufficient organic content (i.e. plankton) and the right temperature (90–160°C), then oil and natural gas can form. After the petroleum is extracted, it is then distilled into fractions to separate it into its constituents (liquefied petroleum gas, gasoline, jet fuel, kerosene and diesel). These petroleum products are then used to produce many products including asphalt, nitrogen fertilisers and plastics. We use plastics in everything from our cars (up to ~20%), synthetics such as nylon, acrylic, polyester. So, our petroleum-based society literally runs on plankton!

Fig. 1.3. The hyperiid amphipod Phronima: the likely inspiration for the alien in the movie of the same name (photo: Anita Slotwinski).

1.2.2 Plankton shaped early human society

It might seem far-fetched, but plankton also shaped early human society. It was the naturalist Charles Darwin who said that fire was one of the most significant achievements of humanity. To make fire, early humans used flint: a type of rock commonly formed from silica-rich plankton such as diatoms and radiolarians. Flint was used for many stone age tools, including weapons, but it was the ability of a flint edge to produce sparks when struck against the rock pyrite that was revolutionary. The ability to produce fire provided early hominids protection from predators, a method for hunting, the ability to cook food, and the capacity to expand activities into the darker and colder hours of night. These cultural innovations changed our diet and behaviour, allowing humans to disperse across the world. Without the ability to use flint formed from plankton to produce fire, it is difficult to see that human society would be where it is today.

1.2.3 Plankton in the movies

Plankton and Karen are well known characters to those who enjoyed the animated TV series SpongeBob SquarePants. Plankton is a copepod, while Karen is his supercomputer. But when sorting through your plankton samples and seeing what different plankton species look like close up, you’ll realise that plankton has probably inspired many characters in the movie industry.

One of the most iconic movie monsters in film history – the antagonist in the film ‘Alien’ – is thought to be inspired by plankton. Phronima is a large planktonic amphipod, up to ~40 mm in size with a head like a praying mantis insect. It has huge eyes and large predatory arms (Fig. 1.3). However, the most remarkable aspect is that some species parasitise salps (Box 1.1): translucent barrel-shaped plankton. Phronima uses the salp for protection and as a flotation device, swimming it through the water, feeding on its host and other plankton as it goes, and rearing its young inside. Finally, the young eventually emerge from the salp, like the Alien erupted from a human host in the movie!

1.2.4 Amazing single-celled plankton inspire architects and engineers!

The structure of many plankton groups such as diatoms, coccolithophores, silicoflagellates, tintinnids, radiolarians, foraminiferans and acantharians are wildly elegant and diverse, being spinose, ribbed, geometric, geodesic, perforated, fluted, ornamented and stellate. This variety of forms was made famous by early ecologists and palaeontologists such as Earnst Haeckl in 1904 in his Kunstformen der Nature (‘Art Forms of Nature’). These stunning shapes inspired Art Nouveau architecture and design, including René Binet’s design for the Printemps department store. Such diverse planktonic forms are now inspiring architects and engineers through biomimetics (Pohl and Nachtigall 2015). Biomimetics is the field of imitation of natural systems and elements to solve human problems. The similarity between how nature solves problems and how architects do can be illustrated by the central dome of Galleria Vittorio Emmanuele in Milan, Italy. This glass dome is supported by a structure of radial and concentric ribs structurally and functionally reminiscent of the valve of the radial centric diatom Arachnoidiscus (Fig. 1.4).

Box 1.1 Salps, larv...