Marine Organisms

Learning Outcomes:

1. Understand the role and types of plankton in aquatic ecosystems.
2. Identify the factors influencing phytoplankton biodiversity and their importance in the carbon cycle.
3. Recognize the role of zooplankton in marine food webs and ecosystem health.
4. Describe the significance, functions, and threats to sea grasses and seaweeds.

Plankton

Plankton encompasses the diverse organisms floating in the surface waters of rivers, lakes, and oceans. This group includes microscopic plants like algae (phytoplankton) and animals such as crustaceans and protozoans (zooplankton). They inhabit nearly all aquatic ecosystems, except for some fast-flowing waters. Their locomotion is limited, making their distribution heavily dependent on water currents. Tropical waters, especially mangrove areas, showcase high growth rates, productivity, and diversity of plankton.

Phytoplankton

Phytoplankton, derived from Greek “phyto” (plant) and “plankton” (drifting), are microscopic plant organisms found in both saline and freshwater environments. They encompass various types like cyanobacteria, silica-encased diatoms, dinoflagellates, green algae, and coccolithophores. These organisms play a crucial role in oxygen production and the carbon cycle.

1. Characteristics:

• Possess chlorophyll to capture sunlight for photosynthesis, consume carbon dioxide, and release oxygen.
• Include bacteria, protists, and predominantly single-celled plants.
• Their biomass exceeds that of terrestrial plants, serving as “pasture grounds” in oceans.

1. Distribution:

• Occupy the euphotic zone of seas and oceans, thriving where light intensity suffices for photosynthesis.
• Highest concentrations are at high latitudes and upwelling areas on continental shelves, with lower concentrations in tropics and subtropics.

Factors Affecting Phytoplankton Biodiversity

1. Light: Restricted to the ocean’s uppermost layers where light intensity supports photosynthesis. The rate of photosynthesis varies with light intensity.
2. Nutrients: Require inorganic nutrients like nitrogen and phosphorus for growth and reproduction. Specific types like diatoms also need silicate (SiO₂). Low iron concentrations in oceans can limit phytoplankton growth.
3. Temperature: Influences photosynthetic production, generally increasing the rate with rising temperatures but decreasing sharply beyond a certain point. Seasonal variations in temperate latitudes affect production rates.
4. Salinity: Alongside light and temperature, salinity impacts primary production.
5. Grazing by Zooplankton: Zooplankton’s grazing rate significantly affects the phytoplankton’s standing crop size and production rate.
6. Distribution: Influenced by wind, waves, nutrients, temperature, salinity, and exposure to UV-B radiation. Productivity is limited to the euphotic zone, influenced by wind and waves.

Important Note: High exposure to solar UV-B radiation in the tropics and subtropics might affect phytoplankton distribution.

Importance of Phytoplankton

1. The Food Web:

• Foundation of aquatic food webs.
• Primary producers feeding everything from zooplankton to whales.

1. Carbon Cycle and Climate Change:

• Responsible for transferring carbon dioxide from the atmosphere to oceans during photosynthesis.
• A “biological carbon pump” that moves about 10 gigatonnes of carbon annually to the deep ocean.
• Changes in phytoplankton growth can impact global carbon dioxide levels, influencing surface temperatures.

Zooplankton

Zooplankton play an integral role in the aquatic food web, facilitating nutrient recycling and transferring organic matter from primary producers to secondary consumers like fish. They are more prevalent in mangrove waterways than in adjacent coastal waters. Their quality and species diversity are key indicators of an ecosystem’s productivity, fertility, and health.

1. Distribution and Abundance:

• Abundant in mangrove waterways, where many juvenile fish are zooplanktivorous.
• Their population determines the quantum of fish stock in an ecosystem.

Important Note: Zooplankton communities reflect the ecosystem’s productivity, aiding in fisheries resource assessment.

Sea Grass

Sea grasses are marine angiosperms resembling grass. They produce flowers, have strap-like or oval leaves, and possess root systems. Sea grasses grow in shallow, calm coastal waters with sandy or muddy bottoms, being the only higher plants fully adapted to saltwater.

1. Distribution:

• Commonly found along the southeast coast of Tamil Nadu, Lakshadweep lagoons, and some regions around the Andaman and Nicobar Islands in India.
• Thrive in areas with high salinity, water clarity, and sandy substratum.

1. Functions:

• Physically reduce wave and current energy.
• Filter suspended sediments and stabilize bottom sediments to prevent erosion.
• Provide habitat for marine invertebrates and fish.
• Act as nutrient sinks near reef flats and estuaries, buffering and filtering nutrients and chemicals.

Important Note: Sea grasses significantly contribute to lagoon ecosystems by supporting fish and migratory bird populations.

Threats to Sea Grass Beds

1. Eutrophication: Excessive nutrients in water bodies lead to dense plant growth and oxygen depletion.
2. Siltation: Increased sediment deposit can bury sea grass beds.
3. Trawling: Disturbs the seabed, destroying sea grass.
4. Coastal Engineering: Constructions can alter water flow and habitat conditions.
5. Over-exploitation: Harvesting for commercial use impacts sea grass populations.

Concept: IUCN prioritizes sea grass conservation due to their ecological importance and the declining populations of species like the Dugong, which depend on sea grasses for food.

Management

1. Mapping: Identify and map major sea grass beds for preservation.
2. Controlled Dredging: Conduct dredging activities far from sea grass beds to prevent siltation and turbidity.

Seaweeds

Seaweeds are macroscopic algae without differentiation into true tissues (roots, stems, and leaves). They attach to rocks, corals, and submerged substrates in the intertidal and shallow subtidal zones.

1. Classification: Based on pigmentation, seaweeds are categorized into blue-green, green, brown, and red varieties.
2. Functions:

• Serve as food for marine organisms.
• Provide habitat for fish breeding grounds.
• Act as sediment sources.

1. Uses:

• Food for humans and feed for animals.
• Fertilizer for plants.
• Provide extracts (agar-agar, alginates, iodine) used commercially.
• Biodegradation produces economically valuable gases like methane.
• Extracts exhibit antibacterial properties.
• Indicators of coastal ecosystem pollution due to their metal-binding and accumulating abilities.

1. Harmful Effects:

• Rotting seaweed emits hydrogen sulfide, a toxic gas that can cause health issues like vomiting and diarrhea.

Threats to Seaweeds

1. Similar to those faced by sea grasses, including eutrophication, siltation, and over-exploitation.

Important Note: Seaweeds act as pollution indicators, especially for heavy metal pollution in coastal ecosystems.

Comparative Overview

Concept: Seaweeds and sea grasses, despite similarities, have distinct roles and ecological functions within marine environments.

Did you know? The five sea turtle species nesting on Indian coasts are Leatherback, Green, Olive Ridley, Hawksbill, and Loggerhead turtles.

Important Note: Sea snakes, typically found in the Indian and Pacific Oceans, have adapted to lay eggs on land or give birth in water, depending on the species.

Do you know? The “ampullae of Lorenzini” is a sensory organ in sharks used to detect electrical fields from prey.

Interesting Fact: Silverfish, an insect found in old books, demonstrate the surprising variety of organisms influenced by marine ecosystems.

IUCN: Has accorded high priority to conserve sea grass due to its importance in marine ecosystems.

Sea Grass Note: Out of 58 global species, 14 species of sea grasses are recorded along the Indian coast.

Endangered Species: Dugong, a marine mammal reliant on sea grass for food, faces extinction.

Threat Mitigation: Mapping and preservation of sea grass beds are crucial, alongside controlled dredging to maintain ecosystem balance.

Conservation Priority: Both sea grasses and seaweeds require active management to mitigate threats from human activities and environmental changes.

Impactful Role: The collective roles of plankton,