Mortality and loss processes in phytoplankton

Mortality and Loss Processes in Phytoplankton

Phytoplankton are members of the autotrophic that that are usually found on top-most parts of bodies of water. They are oftentimes floating over the seas or rivers. The name itself comes fro a Greek word "phyton" which literally means "drifter" or "wanderer." Phytoplankton could not really be seen by the naked eye especially if there are little amount floating in. However, if there grouped in high numbers, phytoplankton usually appears as a green discoloration in the water because of the chlorophyll contained in the cells (Thurman, 1997).

Phytoplankton are normally being compared to the plants because of its chlorophyll contents its means of obtaining energy (the process of photosynthesis). Because of these plant-like characteristics, phytoplankton are often found in areas where there would be enough sun shine, hence they are mostly seen in the surfaces of oceans, seas or lakes. Also, because of the process of photosynthesis that phytoplankton are capable of doing, like plants, phytoplankton have become responsible in the production of enough oxygen in the earth's atmosphere. The cumulative fixation on the primary production of carbon compounds rendered by phytoplankton has also resulted to the continuation of oceanic and freshwater food chain (Thurman, 1997).

Problem Statement

Indeed, phytoplankton, however tiny they may seem, play a very significant, role in the maintenance of balanced ecosystem. The importance of phytoplankton have already been established. However, the reasons for mortality and production and/or continuous reproduction of phytoplankton have yet to be determined.

This paper is generally aimed at answering the problem statement: How do phytoplankton survived and reproduce in today's time? Specifically, the problems that will be analyzed are:

What are phytoplankton and why are they significant?

What is the current statistics of phytoplankton? The mortality rate?

What could possible threaten the continuing existence of phytoplankton?

What should be done to remove the barriers of phytoplankton existence?

Significance of the study

This result of this study will be significant to science students and policy makers. Through this study, students will be given enough background information regarding phytoplankton and the role they provide for mankind. Not only the students can lean the value of phytoplankton, because of this study, they too, will realize the best possible course of actions on how to keep the population of phytoplankton at a manageable rate.

Meanwhile, policymakers, who will be informed of the results of this study, will be given enough idea on what resolutions they can offer so that phytoplankton will be saved and retained as part of the environment. Policymakers will then know what kind of rules or guidelines to implement, how to facilitate it and who will benefit from each guideline.

Lastly, this paper will play a pivotal role for other future researchers who may be interested in the study of phytoplankton. This research can serve as a guide or as a reference material for other researchers.

Methodology

This paper did some in-depth literature review regarding phytoplanktons. Credible journals, articles and scientific reports have been closely studied so as to find ample information regarding the mortality and loss process of phytoplankton. At the same time, same method is done so as to find proven methods on how phytoplankton can be conserved.

Review of Related literature

Phytoplankton composition

Almost all types of phytoplankton are considered as photoautotrophs. However, there are also some types which are mixotrophic in nature, while others are non-pigmented species that are actually heterotrophic. Among these, the best known phytoplankton are the so-called dinoflagellate genera such as Noctiluca and Dinophysis which can obtain organic carbon by ingesting other organisms or detrital material (Thurman, 1997).

With regards to the number of population, the considered most important groups of phytoplankton include the diatoms, cyanobacteria and dinoflagellates, although many other groups of algae are represented. One group of phytoplankton, the coccolithophorids, is somewhat responsible for the release of significant amounts of dimethyl sulfide (DMS) into the atmosphere. In oligotrophic oceanic areas, such as the Sargasso Sea or the South Pacific, small sized cells, called picoplankton, dominated the phytoplankton population. This type is composed of cyanobacteria (Prochlorococcus, Synechococcus) and picoeucaryotes such as Micromonas (Thurman, 1997).

Factors Affecting Growth of Phytoplankton

Phytoplankton are highly dependent on nutrients for growth. They seek and stay in swampy areas or aquatic systems where there is abundance of available nutrients. In fact, the nutrient levels affect the total biomass of phytoplankton, the taxonomic composition, and even the distribution of size of the community, especially in lakes (Pick and Lean 1987). The kinds of macro nutrients phytoplankton usually seek for are nitrate, phosphate or silicic acid. It was noted that the availability of these nutrients is administrated by the balance between the biological pump and upwelling of deep, nutrient-rich waters (Thurman, 1997). However, when it pertains to larger bodies of water, such as the ocean, phytoplankton are also limited by the availability of the micronutrient iron. Hence, there are some scientists who have suggested that iron fertilization could be an alternative course of action to accumulate anthropogenic carbon dioxide (CO2) in the atmosphere (Thurman, 1997).

Other factors that may affect the growth of phytoplankton are the physical features of the area. Like for example in lakes, the lake morphometry, circulation patterns, and the presence and/or absence of thermal stratification. Factors such as these are believed to be affecting the community structure of phytoplankton. It was noted that lakes with thermal stratification are essentially different from those without thermal stratification in a way that shallow lakes without thermal stratification tend to have higher phytoplankton biomass than deep lakes even with similar levels of nutrients (Pridmore et al. 1985). Likewise, the circulation patterns of the lakes affect the phytoplankton biomass because patterns of circulation is believed to be responsible for strong seasonal and within-lake differences in plankton communities (Straskraba 1980).

Predation also affects the growth of phytoplankton because at most times they are dependent on it. Like for example the Cladocera which have high grazing rates. Cladocera can "clear" the water column of algae especially if they are abundant in the area (Lampert et al. 1986). Other kinds of predators such as larval insects and planktivorous fish also have direct influence on the phytoplankton biomass and composition either by altering zooplankton community structure or minimizing the total zooplankton biomass (Vanni and Findlay 1990). Planktivorous fish have the capacity to reduce the number of large-scaled zooplankton, such as Daphnia spp.. This, in turn, enables the release of phytoplankton from grazing pressure thereby increasing the phytoplankton biomass. However, this predation factor could not be used to generalize the effect to phytoplankton because there are some fish who failed to demonstrate the same effect (DeMelo et al. 1992). Also, there are other fish components, such as the 'trophic cascade' (Carpenter et al. 1985) that affects the growth and existence of phytoplankton communities. This is because fish and/or zooplankton manipulations can be accompanied by changes in nutrient availability (McQueen et al. 1992) and thermal structure (Mazumder et al. 1990).

Temperature also affects the growth of phytoplankton. Scientists discovered that in temperate lakes, the seasonal thermal stratification of the water column can significantly influence the composition of epilimnetic phytoplankton by changing the relative sedimentation rates of individual taxa (Reynolds 1984). Meanwhile, in stratified waters, larger cells such as the diatoms, have the tendency to settle out more rapidly than in a mixed or non-stratified water column. This then result to the size distribution of phytoplankton which is shifting toward small or motile cells. Small cells with high growth rates or those with strong buoyancy control generally dominate.

Moreover, thermal stratification and the ratio of nitrogen to phosphorus can predict the distribution and occurrence of specific phytoplankton taxa (Jensen et al. 1994). This stratification can also affect the level of predation refuge for zooplankton, which normally allows zooplankton to control the phytoplankton populations (Wright and Shapiro 1990). Mean depth, which is found to be a closely related factor to thermal stratification, can modify the rate at which abiotic factors affect the phytoplankton community (Alvarez Cobelas et al. 1994). This is also proven to be a key element in the development of coastal phytoplankton blooms (Koseff et al. 1993).

There are actually two groups of phytoplankton, namely:

fast-growing diatoms (the group which do not have the capacity to thrust themselves through the water) flagellates and dinoflagellates (the type of phytoplankton which are able to migrate vertically in the water column in response to light).

Each of the group stated above vary in cell shapes, designs and even ornamentations. However, there is one common denominator among these groups of phytoplankton. All of these are dependent with the oceanic currents for transport to areas that are appropriate for their survival and growth.

This aspect alone clearly depicts that phytoplankton are affected by physical processes in determining the distribution of phytoplankton species (Karl, et al., 2001).

Importance of Phytoplankton

Rapid cell division and population growth in phytoplankton produce millions of cells per liter of seawater. This then result to visible blooms or most commonly called as the "red tides." Because of the ability to reproduce in large amounts in a small amount of time, phytoplankton are considered as the first link in the food chain of nearly all marine animals. Phytoplankton provide food for a large variety of organisms, including the microscopic animals (such as the zooplankton), bivalve molluscan shellfish (like mussels, oysters, scallops, and clams), and small fishes (such as anchovies and sardines). To continue the food chain, these group of animals then provide their own kind of food to other group animals like crabs, starfish, fish, marine birds, marine mammals, and humans (Karl, et al., 2001).

Figure 1. Sample food chain involving phytoplankton

Source: (www.planktonfyi.com/images/foodchain.jpg,2006).

Mortality Rate of Phytoplankton

It was recorded that from 1980's to the present, phytoplankton have been continuously increasing in frequency and distribution worldwide. The reason for such continuing increase in biomass is yet to be determined, but scientists have provided several assumptions including (Karl, et al., 2001):

continuously increase of nutrient input to coastal areas because of massive and productive human activities

Climactic changes that are in large-scale forms (such as global warming)

Movement or transport of toxigenic species in ship counterbalanced water

Improved use of coastal resources

Increased surveillance positive intervention of the government health agencies and researchers.

Figure 2. Biomass distribution of phytoplankton

Source: (www.astro.temple.edu/~sanders1/balance.gif,2006)

Figure 3. Generalized Seasonal Changes in Phytoplankton, Sunlight and Dissolved Nutrients

Source: (www.astro.temple.edu/~sanders1/balance.gif,2006)

Figure 4. Generalized Changes in Phytoplankton Biomass

Source: (www.astro.temple.edu/~sanders1/balance.gif,2006)

Biodiversity of Phytoplankton

Despite continued human intervention and technological advancements, which sometimes hindered most marine animals from continuously reproducing, he phytoplankton have survived and have been to be continuously increasing in mortality rate.

In fact, reports revealed that there have been and increasing diversity of marine phytoplankton (Anya, 1996).

This continued diversity in marine phytoplankton can be attributed to their ability to adapt with the environment, however frequent the changes may occur. There are significant deviations in the environment where phytoplankton live and through time, different species have responded differently to changes in the environments (Anya, 1996).

Over time, scientist have been noting how phytoplankton are able to adapt with the evolutionary changes from oceans to calm waters, even from increasing numbers of predators to changing vertical gradients in the light intensity and nutrients which are essential means of growth. Each phytoplankton have been redefining itself and capitalizing on different combinations of the said factors, thereby creating a new and unique shape, size, and physiology helping them for their survival. There are specific types of phytoplankton which are able to response quickly to the ever-changing requirement of the environment. Some phytoplankton are able to develop their own flexible response to the environment. Also there are instances wherein phytoplankton are able to develop high forms of tolerance to a wide range of possible habitats, while other phytoplankton's requirements are more specific. Because of these variations, the quality and forms of phytoplankton existing nowadays are believed to be enormously diverse species assemblage (Anya, 1996).

Logically, these diversions would result to changes in oceans creating uniformity across wide areas. However the diversity of species within phytoplankton can potentially threaten by those which have inputted heavy amounts of nutrients as caused by sewage or large-scale industrial pollution (Anya, 1996).

The abilities of phytoplankton to reproduce, adapt and change over time despite the hindrances and difference in the environment also vary to the type of phytoplankton there is. There are two major groups of large ultra-phytoplankton - the dinoflagellates and the diatoms (which have been showing signs that they can adapt very differently to ocean dynamics). It must be noted that dinoflagellates are considered as one of the most peculiar groups in the ocean. Dinoflagellates are commonly described to have a size one-fifth of a millimeter in diameter, which contains membrane-covered organic shell. Dinoflagellates are most commonly seen on waters because here, they can gather at the surface to photosynthesize and then plunge down again to swallow down the nutrients presented in the deeper water levels. Although dinoflagellates are usually solitary in nature, they are still able to form chains of cells that swim as a unit (Anya, 1996).