Download Ecology of Plankton - Enviromental Sciences - Lecture Notes and more Study notes Environmental Science in PDF only on Docsity!
ECOLOGY OF PLANKTON
Key words: Phytoplankton, Producer, Productivity
Fig ā 19.
INTRODUCTION
Plankton are those organisms which live suspended in the water of seas, lakes, ponds, and rivers, and which are not able to swim against the currents of water. This latter feature distinguishes plankton from nekton, the community of actively swimming organisms like fish, larger cephalopods, and aquatic mammals. Plankton range in size from ca. 0.2 gm to several meters (large jellyfish), but only the small ones have been the objects of intensive research, the Antarctic krill being the only well-studied plankton organism of > 5 mm.
Plankton form complex biotic communities which are functionally as diverse and show the same richness of interaction as terrestrial communities. Plankton are defined by their ecological niche rather than their phylogenetic or taxonomic classification. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish.
The name plankton is derived from the Greek adjective planktos , meaning "errant", and by extension "wanderer" or "drifter". By definition, organisms classified as plankton are unable to resist ocean currents. While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by the surrounding currents. This is in contrast to nekton organisms that can swim against the ambient flow and control their position (e.g. squid, fish, and marine mammals).
Within the plankton, holoplankton spend their entire life cycle as plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are only planktic for part of their lives (usually the larval stage), and then graduate to either the nekton
or a benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish. Plankton abundance and distribution are strongly dependent on factors such as ambient nutrients concentrations, the physical state of the water column, and the abundance of other plankton. The study of plankton is termed planktology and individual plankton are referred to as plankters.
Functional groupings:
ļ§ Phytoplankton (from Greek phyton , or plant),
Autotrophic, prokaryotic or eukaryotic algae that live near the water surface where there is sufficient light to support photosynthesis. Among the more important groups are the diatoms, cyanobacteria, dinoflagellates and coccolithophores.
ļ§ Zooplankton (from Greek zoon , or animal),
Small protozoans or metazoans (e.g. crustaceans and other animals) that feed on other plankton and telonemia. Some of the eggs and larvae of larger animals, such as fish, crustaceans, and annelids, are included here.
ļ§ Bacterioplankton ,
Bacteria and archaea, which play an important role in remineralising organic material down the water column (note that the prokaryotic phytoplankton are also bacterioplankton).
Fig ā 19.
This scheme divides the plankton community into broad producer , consumer and recycler groups. However, determining the trophic level of some plankton is not straightforward. For example, although most dinoflagellates are
Because of the central role of plankton in the functioning open-water foodwebs and ecosystems, plankton ecology has always been a core discipline of limnology and biological oceanography. Beyond their importance for aquatic systems, plankton are most suitable model organisms for classic topics of general ecology, such as competition, predator-prey relationships, food-web structure, succession, transfer of matter, and energy. Small size, rapid population growth (doubling times < 1 day for bacteria and small phytoplankton to several days or weeks for zooplankton), high abundances (millions per ml for bacteria, millions per 1 for phytoplankton), and a relatively homogeneous distribution in their environment facilitate field and experimental studies. Processes which take years to centuries in terrestrial systems, like competitive exclusion and succession, take only weeks in plankton.
The phytoplankton:
The plantlike community of plankton is called phytoplankton, and the animal-like community is known as zooplankton. This convenient division is not without fault, for strictly speaking, many planktonic organisms are neither clearly plant nor animal but rather are better described as protists. When size is used as a criterion, plankton can be subdivided into macroplankton, microplankton, and nannoplankton, though no sharp lines can be drawn between these categories. Macroplankton can be collected with a coarse net, and morphological details of individual organisms are easily discernible. These forms, one millimetre or more in length, ordinarily do not include phytoplankton. Microplankton (also called net plankton) is composed of organisms between 0.05 and 1 mm in size and is a mixture of phytoplankton and zooplankton. The lower limit of its size range is fixed by the aperture of the finest cloth used for plankton nets. Nannoplankton (dwarf plankton) passes through all nets and consists of forms of a size less than 0.05 mm. Phytoplanktonic organisms dominate the nannoplankton.
Size groups:
Group Size range (ESD) Megaplankton > 2x10-^2 m (20+mm) Metazoans; e.g. jellyfish; cteanophores; salps and pyrosomes (pelagic Tunicata); Cephalopoda Macroplankton 2x10-^3 ā 2x10-^2 m (2- 20 mm) Metazoans; e.g. Pteropods;Chaeto gnaths; Euphausiacea(krill); Mesoplankton 2x10-^4 - 2x10-^3 m (0.2mm-2mm) Metazoans;e.g.copepods; Medusa e; Cladocera; Ostracoda Microplankton 2x10-^5 - 2x10-^4 m (20-200Ī¼m) large eukaryotic protists;most ph ytoplankton; Protozoa (Foraminif era); ciliates; Rotifera;juvenile m etazoans - Crustacea (copepod nauplii) Nanoplankton 2x10-^6 - 2x10-^5 m (2-20 Ī¼m) small eukaryotic protists;Small D iatoms;Small Flagellates; Pyrroph yta; Chrysophyta; Chlorophyta; Xanthophyta Picoplankton 2x10-^7 - 2x10-^6 m (0.2-2 Ī¼m) small eukaryotic protists; bacteria ; Chrysophyta
Femtoplankton < 2x10-^7 m (<0.2 Ī¼m) marine viruses
However, some of these terms may be used with very different boundaries, especially on the larger end of the scale. The existence and importance of nano and even smaller plankton was only discovered during the 1980s, but they are thought to make up the largest proportion of all plankton in number and diversity. The microplankton and smaller groups are microorganisms and operate at low Reynolds numbers, where the viscosity of water is much more important than its mass or inertia.
The chief components of marine phytoplankton are found within the algal groups and include diatoms (see phytoplankton video), dinoflagellates and coccolithophorids. Silicoflagellates, cryptomonads, and green algae are found in most plankton samples. Freshwater phytoplankton, usually rich in green algae, also includes diatoms, blue-green algae, and true flagellates.
The zooplankton:
Fig ā 19.
The zooplankton is divided into two groups. Temporary plankton consists of planktonic eggs and larvae of members of the benthos and nekton; permanent plankton includes all animals that live their complete life cycles in a floating state. The temporary plankton, particularly abundant in coastal areas, is characteristically seasonal in occurrence, though variations in spawning time of different species ensure its presence in all seasons. Representatives from nearly every phylum of the animal kingdom are found in the permanent plankton. Among the protozoans, planktonic foraminiferans and radiolarians are so abundant and widespread that their skeletons constitute the bulk of bottom sediments over wide ocean areas. They are absent in fresh water. The ciliate protozoans are represented mainly by the tintinnids, which are between 20 and 640 microns in size and sometimes occur in vast numbers. Among the planktonic coelenterates are the beautiful siphonophores ( e.g., Physalia, the Portuguese man-of-war) and the jellyfishes. Planktonic ctenophores, called comb jellies, or sea walnuts, are also common. Freshwater rotifers may be present in plankton in vast numbers during the warmer seasons. A group of organisms that can be found
situation exists in the Black Sea, where water below 130ā180 m contains hydrogen sulfide and no oxygen. Under these conditions only bacteria are found. Ecosystems consist of populations, which in turn consist of individuals that interact with one another and with the environment. Biological interactions in the ocean are not between populations or between trophic levels, as many box-model representations of pelagic food webs might lead us to think. Trophic levels and populations are abstractions, and interactions occur at the level of the individual. āBlindā sampling of bulk properties may result in observed distributional patterns, for example, that cannot be understood and explained from such an approach on its own. The picture must be complemented by approaches that consider the individual in its immediate environment and that provide a mechanistic understanding of the functioning of individuals and of components of the larger systems.
This allows us to build models and to extrapolate observations beyond the system in which the observations were made. Traditionally, scientists who go on cruises and examine distribution patterns of both biota and environmental properties using sampling are considered biological oceanographers, and those who explore the functioning of individuals, for example by conducting laboratory experiments with organisms, are considered marine biologists. We need to combine the two approaches to understand the ecology of the oceans.
The motivation to try to understand the ecology of planktonic organisms is twofold. The first driving force has to do with a simple interest in natural history. It is fascinating to watch the behavior of live plankters under the microscope orābetterāfree- swimming plankters by video; they have different but often beautiful forms and colors, and even closely related species may behave very differently, which makes identifying live plankton much easier than identifying dead ones. The second reason for examining the adaptations and behavior of plankters is our interest in understanding overall properties of pelagic systems and how the pelagic system relates to the larger- scale issues of fisheriesā yield, CO 2 balance, global climate, and others. Understanding the mechanistics of individual behaviors and interactions may allow us to predict rates and to scale rates to sizes, which, in turn, may help us understand the (size) structure and function of pelagic systems and to predict effects of environmental changes and human impacts.
The Encounter Problem:
Life is all about encounters. In the ocean, for example, phytoplankton cells need to encounter molecules of nutrient salts and inorganic carbon; bacteria need to encounter organic molecules; viruses need to encounter their hosts; predators need to encounter their prey; and males need to encounter females (or vice versa). Other important processes in the ocean, such as the formation of marine snow aggregates, likewise depend on encounters, here encounters between the component particles.
All organisms, including plankters, have three main tasks in life, namely to eat, to reproduce, and to avoid being eaten, all related to encounters or avoiding encounters. The behavior, morphology, and ecology of planktonic organisms must to a large extent represent adaptations to undertake these missions, and the diversity of form, function, and behavior that we can observe among plankters must be the result of different ways of solving the problems in the environment in which they live.The pelagic environment seen from the point of view of a small plankter is very different from the environment experienced by humans, and our intuition is often insufficient to allow us to understand the behavioural adaptations of planktonic organisms. Thus, although ornithologists to a large extent may be able to
understand the behavior of their study organisms by using common sense, copepodologists rarely can, to rephrase the title of a classical ecology paper (Hutchinson 1951). For example, at the scale of planktonic organisms, the medium is viscous, and inertial forces therefore are insignificant, which makes moving an entirely different undertaking than what we as humans are used to or have seen other terrestrial animals do; the density of water is orders of magnitude higher than the density of air, which makes floatation easier and currents more important; for the smallest pelagic organisms (bacteria), thermally driven Brownian motion makes steering impossible; and most plankton use senses different from, and less far-reaching than, vision to perceive the environment. In addition, the pelagic environment is three- dimensional, whereas humans mainly move in only two dimensions. This implies, among other things, which average distances between a planktonic organism and its target may be very large, maybe thousands of body lengths. Because of the often non-intuitive nature of the immediate environment of small pelagic organisms, we need to appeal to fluid dynamic considerations in order to achieve a mechanistic understanding of the small- scale interactions between plankters and their environment.
In pursuing the encounter problem we can write a very general equation that describes encounter rates
E = Ī²C1C2 (1.1a)
where E is the number of encounters happening per unit time and volume between particle types 1 and 2, C1 and C2 are the concentrations of these particles, and Ī² is the encounter rate kernel (L3T ā 1). Often we are interested in looking at the per capita rate, that is, the rate at which one particle of type 1 encounters a particle of type 2:
e = E/C1 = Ī²C2 (1.1b)
For example, if particle 1 is a suspension-feeding ciliate and C2 the concentration of its phytoplankton prey, then Ī² is the ciliateās clearance rate, and e its ingestion rate (assuming that all encountered particles are ingested). The clearance rate is the equivalent volume of water from which the ciliate removes all prey particles per unit time. In many suspension- feeding ciliates, the clearance rate can be interpreted directly as a filtration rate; that is, the rate at which water is passed through a filtering structure that retains suspended particles. As a different but similar example: if particle 1 is a fish larva looking for food, and particle 2 its microzooplankton prey, then Ī² is the volume of water that the larvae can search for prey items per unit time; if all encountered prey are consumed, then e is the ingestion rate of the fish larva. We may also see the process from the point of view of the prey, in which case Ī²C is the mortality rate of the phytoplankton or microzooplankton prey population through ciliate grazing or fish larval feeding. As a final example: if C1 is the concentration of bacteria, and C2 the concentration of organic molecules on which the bacteria feed, then e is the assimilation rate; it is more difficult to give a physical interpretation of Ī² in this case. However, it is, like a clearance rate, the imaginary volume of water from which the bacterium removes all molecules per unit time. In fact, any encounter problem can be cast in terms of the general equation (eq. 1), but obviously the interpretation or meaning of the terms may be very different. The processes or mechanisms responsible for encounters are contained in the encounter-rate kernel. Obviously, from the examples above, these mechanisms are diverse. Intuitively, encounter rates depend on two factors: the motility of the encountering āparticlesā and the ambient fluid motion that may enhance encounter rates. Motility encompasses here the diffusivity of molecules, the swimming of organisms, and the sinking of particles. In
As a human resource, plankton has only begun to be developed and exploited. It may in time be the chief food supply of the world, in view of its high biological productivity and wide extent. It has been demonstrated on several occasions that large-scale cultures of algae are technically feasible. The unicellular green alga Chorella has been used particularly in this connection. Through ample culture conditions, production is directed toward protein content greater than 50 percent. Although this protein has a suitable balance of essential amino acids, its low degree of digestibility prevents practical use. Phytoplankton may become increasingly important in space travel as a source for food and for gas exchange. The carbon dioxide released during respiration of spacecraft personnel would be transformed into organic substances by the algae, while the oxygen liberated during this process would support human respiration.
