Limnology (2+1)

Unit 6- Planktonic organisms

6.3. Food for plankton organisms
Plankton contains miscellaneous assemblage of organisms which are fairly high in animal scale, food requirement are diverse and can be considered best in connection with individual group.
Green phytoplankton: Chlorophyll bearing phytoplankton utilise organic and inorganic materials dissolved in water. Certain chlorophyll bearing Protozoa utilize certain amount of particulate materials.
Non green phytoplankton: They do not possess chlorophyll such as bacteria depend upon dissolved materials.
Protozoa: The protozoan plankters feed upon minute algae and bacteria. Utilisation of dissolved substances has been demonstrated in certain protozoa.
Metazoan – Neumann (1929) considered the relation of zooplankton as seston feeders into the 4 types viz.
1. grasping type : such as Rhizopoda which touch and secure seston with pseudopodia
2. filtration type : filtering of seston from water as animal moves
3. sedimentation type : capture of seston by means of induced water currents and
4. predatory type : capture of other organisms
Cladocera: They obtain food by active filtration of water and by predation. Cladocerans filter their food from water mainly particulate matter such as inorganic debris, organic debris and living organisms. Experiments showed that Daphnia and Bosmina took ordinary pond debris, algae, carmine and finely divided humus indiscriminately filling the digestive tract with mixture.
Copepods: Thorough examination of the digestive tracts of certain copepods have revealed that
a) Cyclops had fragments of exoskeleton of Entomostraca, jaws of rotifers
b) Diaptomus and
c) Nauplii had eaten finely granular mass for minute algae.
Seasonal Changes of Body Form in Plankton organisms
Morphological character of body form changes
The seasonal change of body form differ greatly in different plankton and the changes from winter to summer forms by way of increase in body surface compared to body volume.
For eg,
a) the flagellate shows a tendency in summer to have a longer stalks
b) the Ceratium hirundinella develops in summer and produce a longer, narrower body from
c) Cladoceran body in summer is higher than long with a longer back and longer posterior spine and is reverse in winter season
d) Rotifers show changes in body size, changes in number and development of anterior spines in pond condition which is less frequently in lakes.
General biological features
The following statements are based largely upon the summary by Wesenberg Lund (1926)
1. Seasonal changes of form is best developed among perennial species during summer and disappear during winter
2. Seasonal changes of form may take a different aspect in different lakes due to local variations
3. Transition ie, transformational stages occur almost abruptly over a period of 2 or 3 weeks in autumn but is gradual from summer to winter periods
4. The body form may not change in different season if it is having same body shape in summer
5. Body form among cladocerans and rotifers is restricted in case of females
Primary productivity
The primary production in the aquatic ecosystem starts with the synthesis of organic compounds from the inorganic constituents of water by the activity of plants / phytoplankton in the presence of sunlight. The inorganic constituents which form the raw material for this synthesis are water, carbon dioxide, nitrate ions, phosphate ions and various other chemical substances. The products are mainly carbohydrates and proteins and fats in very small quantities. Organic production by plants is the first step in tapping energy by living beings from non-living natural resources and hence called primary productivity.
The method of estimating primary productivity by dark and light bottle method was introduced by Garder and Gran (1930). In this method, the water samples are incubated for a certain period in light and dark bottles which are then suspended at the same depths where from the sample are taken. In light bottles, oxygen is released as a result of photosynthesis and a part of oxygen is used for community respiration. In the dark bottles, only oxygen consumption takes place as a result of respiration. The amount of oxygen liberated by phytoplankton during photosynthesis is considered as a measure of primary production.
The total quantity of organic matter synthesized by a unit measure of plants in a unit time is termed as gross primary productivity. Some of this material is broken down by plants themselves for their respiration, excretion and death etc. The reminder which becomes the plant tissue is called net primary productivity.
Factors influencing primary productivity
The rates of photosynthetic primary production by phytoplankton vary greatly in different waters and at different times. The variation in photosynthetic rates suggests that there are factors that differ from place to place and from time to time, which determine the evident differences in photosynthetic activity. The major influencing factors are :
1. Light intensity
The radiant energy that reaches the surface reacts with the dissolved and particulate materials present in the water and this reaction brings about absorption and scattering. Illumination of surface layers varies with place, time, light intensity, water transparency, diurnally, seasonally and attitudinally and also with cloud condition and atmospheric absorption. Depending on the conditions, about 50% of the incident light is reflected back; about 80% of the total radiation entering the surface is absorbed within the upper 10 m. and only about 0.1 to 0.2% is converted into photosynthetic production.
2. Temperature level
Very high temperatures inhibit photosynthesis since they damage the enzymes and cell structure as in photoinhibition. Relatively hot and light surface layer is more vulnerable to turbulent mixing due to which the algae can be carries down below the photic zone. At critical depth, the total primary production in the water column above equals the total loss by respiration in the same column.
3. Nutrient supply
The fundamental importance of nutrients is that the rate at which they are supplied may determine the rate of primary production. The potential limitations of producer activity by nutrients show that after addition of nutrients, net production increases. The productivity of the system is nutrient limited regardless of the changes in species composition that often result from the enrichment.
4. Grazing rate
The biomass of zooplankton generally coincides with minima of phytoplankton density on account of grazing. In some areas of the water bodies, as much as 99.5% of the net primary production may be grazed. The plankton upon death, would liberate phosphorous and nitrogen rapidly in the water making it available to phytoplankton growth.