CHAPTER 2.2 Aquatic ecosystem
Overview of Aquatic Ecosystems
• Two global realms
– Freshwater (lentic, lotic, wetlands)
– Marine (coastal to hadal)
• Continuum is linked by the water cycle: precipitation → runoff → stream → lake → river → estuary → ocean → evaporation.
• Diversity spans microscopic plankton to megafauna (whales); physical and chemical gradients dictate biotic distribution.
Basic Needs of Aquatic Biota
• CO2 for photosynthesis / shell building. • O2 for aerobic respiration.
• Sunlight – drives primary production; defines photic, dysphotic, aphotic zones.
• Nutrients – organic food + inorganic minerals (e.g.
NO3^-, PO4^{3-}, K^+).
Human Impacts on Aquatic Ecosystems
• Provisioning motives: food harvest, transportation routes, hydropower, cooling water, waste disposal.
• Structural alterations: dams, canals, dredging, draining wetlands.
• Recreational pressure: boating, diving, shoreline development.
• Pollution cascades: eutrophication, thermal loading, sedimentation, plastics, heavy metals.
Factors Influencing the Availability of Basic Needs
• Dissolved substances (nutrients, gases).
• Suspended matter (silt, algae) → light attenuation.
• Depth & bathymetry – control pressure, light, temperature.
• Temperature – stratification, metabolic rates.
• Flow regime (lotic vs. lentic) – oxygenation, substrate dynamics.
• Bottom type – muddy/sandy/rocky/organic.
• Internal currents – upwelling, convection.
• Connectivity – isolation limits gene flow and nutrient inputs.
Classification of Aquatic Ecosystems
Freshwater Ecosystems
Standing Water (Lentic – Lakes & Ponds)
• Formed by tectonics, volcanism, glaciation, landslides, ice/biotic/deflation dams, river cut-offs.
• Morphology (area, depth) mirrors origin; substrate initially geological, later biological.
• Key physical drivers: oxygen, temperature, light.
Lake Zonation
• Littoral – shallow, warm, high light & wave action; rooted plants, invertebrates, amphibians.
• Limnetic – open surface water; dominated by phyto- & zooplankton; major trophogenic zone.
• Profundal – cold, dense, aphotic; low O2, high CO2/methane; decomposers.
• Photic – vertical zone of sufficient light for photosynthesis; limited by nutrients, grazing.
• Benthic – sediments; scavengers, decomposers; low photosynthesis = low O_2.
Nutrient Status of Lakes
• Oligotrophic: clear blue-green water, low P, high O2, sandy/rocky bottom, sparse biota. • Mesotrophic: moderate nutrients; stained water; productive littoral; some hypolimnetic O2 loss.
• Eutrophic: high N & P; algal blooms; thick organic sediments; anoxic hypolimnion.
Thermal Stratification
• Epilimnion – \approx upper 6\,\text{m} warm, mixed, high O2. • Metalimnion (thermocline) – rapid T & O2 drop; depth varies with season/latitude.
• Hypolimnion – cold (near 4\,^{\circ}\text{C} in temperate lakes), isolated from wind, low light.
Biota per Layer (example)
• Epilimnion: water striders, plankton, sunfish.
• Thermocline: plankton, cool-water fish.
• Hypolimnion: cold-water fish (trout).
• Benthos: anaerobes, leeches, insect larvae.
Adaptations
• Plants: anchored roots (spike rush), tubular leaves (bulrush), aerating tissues (pond lily), no cuticle in submerged leaves (Chara).
• Fish: laterally compressed bodies for maneuvering among plants (sunfish).
• Insects: carry air bubbles (water boatman, diving beetles).
Running Water (Lotic – Rivers & Streams)
• Flow = gravity-driven from source to mouth; longitudinal gradient shapes habitats.
• Upper course: steep, narrow, V-shaped, cold, high O_2.
• Middle course: meanders, wider, moderate gradient.
• Lower course: wide, gentle, distributaries, high sediment load.
• River age: youthful (steep, few tributaries) → mature (moderate) → old (low gradient, floodplain).
• World’s longest: Nile =4157\,\text{mi}, Amazon =3915\,\text{mi}, Chang/Yangtze =3434\,\text{mi}.
Ecosystem Services
• Biodiversity, fisheries, navigation, hydropower, recreation, nutrient/sediment transport, flood regulation.
Functional Feeding Groups
• Shredders – process CPOM (leaf litter).
• Collectors – filter/gather FPOM.
• Grazers – scrape algal films.
• Gougers – burrow into submerged wood.
Adaptations to Flow
• Plants: holdfasts (Fontinalis moss), cushion colonies.
• Animals: streamlined, flattened bodies; sticky undersides; caddisfly cases.
Transitional Communities
Estuaries
• Semi-enclosed mixing of freshwater & seawater; brackish \text{salinity}<35\,\text{ppt}.
• Nutrient traps via counter-flow; high productivity & biodiversity.
• Ecosystem services: nurseries for fish/shellfish, water purification, flood buffering, storm surge absorption.
• Challenges: fluctuating salinity, maintaining position in soft sediments; most organisms benthic.
Wetlands (Swamp, Marsh, Bog, Fen)
• Saturated soils part of year.
• Swamps – trees (bald cypress), high productivity.
• Marshes – no trees, grasses, high productivity.
• Bogs – precipitation-fed, peat, acidic, low productivity.
• Fens – groundwater-fed, peat, slightly higher nutrients.
• Importance: "nature’s septic tank" – nutrient trap, water filter, aquifer recharge, biodiversity hotspots; threats include eutrophication, draining, construction.
Marine Ecosystems
• Cover 71\% of surface; hold 97\% of water; average salinity 35\,\text{ppt} ((\approx85\%) Na^+ & Cl^-).
Vertical Zonation (Open Sea)
• Coastal zone (over continental shelf) – highest productivity.
• Epipelagic/Euphotic 0-200\,m – photosynthesis; O_2 declines near 200\,m.
• Mesopelagic 200-1000\,m – bioluminescent organisms.
• Bathypelagic 1000-2000\,m; Abyssopelagic 2000-6000\,m; Hadal >6000\,m – high pressure, low T.
• Benthic province mirrors pelagic depths (sublittoral to hadal).
Shorelines
• Rocky coasts: intertidal zone; adaptations to wave force (byssus, holdfast), desiccation, salinity & temperature extremes; stabilize sediments, fish nurseries.
• Sandy beaches: unconsolidated substrate, dynamic; buffer zones against wave attack; habitats for crabs, birds, turtles; human recreation pressure.
Barrier Islands
• Low, narrow, offshore sand ridges parallel to coast; protect mainland from storms; primary & secondary dunes critical; threatened by sea-level rise & development.
Coral Reefs
• Located 30^{\circ}N-30^{\circ}S, warm clear shallow water; built by coral polyps with calcium carbonate skeletons + symbiotic zooxanthellae.
• Ecosystem services: coastal protection, biodiversity (( \approx0.1\%) ocean area yet (>1\text{M}) species), drug discovery, carbon regulation.
• Threats: destructive fishing, aquarium trade, bleaching (thermal stress expelling zooxanthellae), ocean acidification (reduced pH hampers calcification).
• Bleached corals = white, weakened, susceptible to disease; biodiversity loss.
Open Ocean (Pelagic)
• Beyond continental shelf; driven by global currents; supports large schools of fish, sharks, whales.
Cross-Cutting Concepts & Real-World Relevance
• Aquatic ecosystems furnish ecological (climate moderation, CO_2 uptake, nutrient cycling) and economic services (food, energy, tourism).
• Connectivity means upstream actions (e.g., fertilizer runoff) cascade to downstream (eutrophic lakes, dead-zone estuaries, reef decline).
• Conservation strategies: protect riparian buffers, maintain environmental flows, wetland preservation, marine protected areas, reef restoration, barrier island zoning.
• Ethical dimension: stewardship of "blue wilderness" we barely understand – "we know more about the moon than the oceans" underscores research need.