Benthic algae and phytoplankton

Lecture 4

Subtopics

Benthic algae

Benthic algae usually cover hard bottoms from the

seashore down to 20 to 40 m depth, depending on the clarity of the ocean.

Below that level, insufficient sunlight hampers their growth. Where the bottom is sandy or muddy the benthic algae cannot attach themselves as they have no roots.

Benthic algae

In some of these soft bottom areas, where the ocean

is calm enough, marine grasses (Zoostera sp.) are able to live.

These are not algae but higher plants as most plants on land. Apart from this, algae dominate the plant life in the ocean around Iceland.

Benthic algae

Benthic algae are a source of food, energy and cover for many organisms.

Dead algae also drift to the open ocean and are a source of food for detritus and filter feeders in ecosystems further away.

In this way, the productivity of the benthic algae in shallow waters directly or indirectly affects the efficiency of the entire marine ecosystem

Importance of Benthic Algae

The primary production per m2 of the kelp forests is

among the highest in the world, comparable to the jungles of the tropics. Therefore, kelp forests are the highest primary production ecosystems in Iceland per m2, on land or water. But as mentioned before, the kelp forests cover a relatively small area compared to the open ocean where the phytoplankton dominates, and therefore their contribution to total primary production is relatively small.


The algae cover is notably zoned; different species

adapt to different depths or are pushed by competition into marginal zones.

In general, physical factors, such as air temperature, salinity or draught, control what species can live in the upper parts of the seashore, but biological factors, such as competition and grazing, control the species composition further down in the ocean.


A few cods in a kelp forest

Benthic algae are generally split into three main

groups,

1. Green algae (chlorophyta),

2. Brown algae (phaeophyta) and

3. Red algae (rhodophyta), named after the different colouration caused by different pigments in these groups.

Groups of benthic algae

Species from all groups occur at various depths but

in general:

The green algae are most common in the upper part of the shore,

Brown algae in the lower part of the seashore and shallower part of the ocean.

Below that the red algae are the most common.





The brown algae are largest and most conspicuous.

Almost all the large algae species found on the seashore and in shallow waters are brown algae, usually referred to as kelp.

These form kelp forests in the ocean that are structurally similar to forests on land.

As in forests on land, many other species of animals and algae are able to thrive in between the large kelp; many species also live on the kelp branches.


The dominant algae or seaweed species on the Icelandic

seashore are knotted wrach (Ascophyllum nodosum) and several species of the genus Fucus.

Below the seashore, much larger species of the genus Laminariadominate the kelp forests of the North Atlantic (including Icelandic waters).

Further down, where the large kelp species cannot live, coralline algae can cover the bottom; these are red algae.

Over long time-scales dead coralline algae can form thick sediments of calcium which are called mearl beds.


Benthic algae have been utilized in Iceland for

centuries.

They were used as sheep food, fertilizers and for burning in stoves.

They are actually very bad for that, but general lack of other things to burn forced many Icelanders to use kelp for that purpose.

Some species are also edible and provided an important food source in some areas.

Utilization of benthic algae

Benthic algae are rarely eaten today, except as rare exotic food and with sushi that has recently become popular in Iceland as well as in other western countries.

Still about 20,000 t of knotted wrach and 4,000 t of oarweed kelp (Laminaria digitata) are harvested in Breiðafjörður, western Iceland, primarily to produce alginates, feed supplements and fertilizers. Geothermal energy is used in this process.

Recently mearl sediments have been harvested for fertilizers.

Utilization of benthic algae

Phytoplankton

The organisms inhabiting the pelagic zone are grouped

into two broad categories-plankton & nekton.

The word plankton was first of coined by Henson ,which means that which drifts.

Plankton further divided into phytoplankton & zooplankton.

Most phytoplankton are too small to be individually seen with the unaided eye.

When they present in high enough numbers ,they may appear as green discoloration of the water due to the presence of chlorophyll within their cells.

INTRODUCTION

Phytoplankton are photosynthesizing microscopic

organisms that inhabit the upper sunlit layer of almost all oceans and bodies of fresh water. They are agents for "primary production," the creation of organic compounds from carbon dioxide dissolved in the water, a process that sustains the aquatic food web.

Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water.

Ecology

Phytoplankton account for half of all photosynthetic

activity on Earth.

Their cumulative energy fixation in carbon compounds (primary production) is the basis for the vast majority of oceanic and also many freshwater food webs (chemosynthesis is a notable exception).

The effects of anthropogenic warming on the global population of phytoplankton is an area of active research.

Ecology

Changes in the vertical stratification of the water column,

the rate of temperature-dependent biological reactions, and the atmospheric supply of nutrients are expected to have important effects on future phytoplankton productivity.

Additionally, changes in the mortality of phytoplankton due to rates of zooplankton grazing may be significant.

As a side note, one of the more remarkable food chains in the ocean – remarkable because of the small number of links – is that of phytoplankton-feeding krill (a crustacean similar to a tiny shrimp) feeding baleen whales.

Ecology

Phytoplankton are also crucially dependent on minerals.

These are primarily macronutrients such as nitrate, phosphate or silicic acid, whose availability is governed by the balance between the so-called biological pump and upwelling of deep, nutrient-rich waters.

However, across large regions of the World Ocean such as the Southern Ocean, phytoplankton are also limited by the lack of the micronutrient iron.

This has led to some scientists advocating iron fertilization as a means to counteract the accumulation of human-produced carbon dioxide (CO2) in the atmosphere.

Ecology

Phytoplankton include wide variety of

photosynthetic organism like

1) Diatoms

2) Dinoflagellates

3) Cryptomonads

4) Green algae

5) Blue green algae

Out of these diatoms & dinoflagellates are predominate.

Types Of Phytoplankton




The diatoms are enclosed within a box made up of

two parts or valves fitting one over another.

The box is made up of silicon dioxide & is ornamented with species specific designs pits & perforations.

They are dominant in temperate & colder regions.

They may occur singly or may occur in chains.

Diatoms

DIATOMS THALASSIOSIRA

The second major group of phytoplankton is

recognizable by possesssing two flagella.

They use flagella to move themselves through the water.

They lack the external skeleton of silicon but are often armored with plates of the carbohydrates cellulose.

They are solitary , single small organisms.

Dinoflagellates

The microscopic phytoplankton float on or near the

surface of water.

As they are autotrophic components of the plankton community they are responsible for much of the oxygen present in earth’s atmosphere.

The population of phytoplankton in any aquatic system is biological wealth of water for fishes & constitutes a vital link in the food chain.

Phtyoplankton have definite role in proper growth & maintenance of a water body.

Ecological Balance

Phytoplankton is a key food item in both Aquaculture &

mariculture .And it is utilized as food for animals being farmed.

In mariculture , the phytoplankton is naturally occuring& is introduced into enclosures with the normal circulation of sea water.

In aquaculture, phytoplankton must be obtained & introduced directly.

Phytoplankton is also used to feed many varieties of aquacultured molluscs, including pearl oysters.

Importance of Phytoplankton

Science

Benthic algae and phytoplankton