🦕Paleoecology Unit 9 – Marine Ecosystems: Reefs, Benthos, and Pelagic

Key Concepts and Definitions

• Marine ecosystems encompass all living organisms and their physical environment in the ocean
• Biodiversity refers to the variety of life forms within an ecosystem, including genetic and species diversity
• Trophic levels organize organisms based on their position in the food chain (primary producers, primary consumers, secondary consumers, tertiary consumers)
• Nutrient cycling involves the transfer and recycling of essential nutrients (carbon, nitrogen, phosphorus) through biotic and abiotic components of an ecosystem
• Benthic zone comprises the seafloor and its inhabitants, while the pelagic zone includes the water column above the seafloor
• Symbiosis describes close and long-term interactions between different species (mutualism, commensalism, parasitism)
• Keystone species play a disproportionately large role in maintaining the structure and function of an ecosystem (sea otters, sea stars)

Marine Ecosystem Types and Characteristics

• Coastal ecosystems include estuaries, salt marshes, mangroves, and seagrass beds, characterized by high productivity and biodiversity
• Coral reefs thrive in shallow, warm, and clear waters, supporting a wide array of marine life
• Open ocean ecosystems, divided into epipelagic, mesopelagic, and bathypelagic zones, host diverse communities adapted to varying light and nutrient availability
• Deep-sea ecosystems, such as hydrothermal vents and cold seeps, support unique chemosynthetic communities in the absence of sunlight
• Polar ecosystems (Arctic and Antarctic) feature cold-adapted species and seasonal variations in ice cover and primary productivity
• Upwelling regions bring nutrient-rich deep waters to the surface, fueling high primary productivity and supporting abundant marine life
• Seamounts, underwater mountains, create localized currents and upwelling, attracting diverse marine communities

Reef Ecosystems: Structure and Function

• Coral reefs are built by colonial cnidarians that secrete calcium carbonate skeletons, forming complex three-dimensional structures
• Zooxanthellae, symbiotic algae living within coral tissues, provide corals with energy through photosynthesis and enhance calcification rates
• Coral reefs support high biodiversity, with numerous species of fish, invertebrates, and microorganisms
  • Reef fish (parrotfish, butterflyfish) play essential roles in maintaining coral health and ecosystem balance
  • Invertebrates (sponges, mollusks, echinoderms) contribute to reef structure, nutrient cycling, and food web dynamics
• Coral reefs protect coastlines from erosion and storm damage by dissipating wave energy
• Reef ecosystems are sensitive to environmental stressors (ocean acidification, temperature rise, pollution) and can undergo phase shifts to algae-dominated states
• Coral bleaching occurs when corals expel their zooxanthellae due to stress, often leading to coral mortality if prolonged

Benthic Environments and Communities

• Soft-bottom habitats, such as sandy or muddy seafloors, host diverse infaunal (living within sediments) and epifaunal (living on sediment surface) communities
  • Infaunal organisms (polychaete worms, bivalves, crustaceans) play crucial roles in sediment bioturbation and nutrient cycling
  • Epifaunal organisms (sea stars, sea urchins, crabs) contribute to benthic food webs and can influence community structure through predation and competition
• Hard-bottom habitats, including rocky shores and submarine canyons, support sessile (attached) and mobile organisms adapted to strong currents and wave action
• Kelp forests, dominated by large brown algae, provide shelter and food for numerous species (fish, invertebrates, marine mammals)
• Seagrass beds stabilize sediments, improve water quality, and serve as nursery grounds for many marine species
• Benthic-pelagic coupling links benthic and pelagic ecosystems through the exchange of nutrients, organic matter, and organisms

Pelagic Zones and Their Inhabitants

• Epipelagic zone (0-200 m) receives sufficient sunlight for photosynthesis and hosts diverse planktonic and nektonic communities
  • Phytoplankton (diatoms, dinoflagellates) form the base of pelagic food webs and contribute significantly to global primary productivity
  • Zooplankton (copepods, krill, jellyfish) consume phytoplankton and transfer energy to higher trophic levels
• Mesopelagic zone (200-1000 m) is characterized by diminishing light and hosts organisms with adaptations for low-light conditions (bioluminescence, vertical migration)
• Bathypelagic zone (1000-4000 m) is entirely dark and supports organisms adapted to high pressure and scarce food resources
  • Detritivores (amphipods, copepods) consume sinking organic matter from upper layers
  • Predators (anglerfish, viperfish) have specialized adaptations for capturing prey in the dark
• Pelagic fish (tuna, billfish, sharks) are highly mobile and play important roles as top predators in open ocean ecosystems
• Marine mammals (whales, dolphins, seals) inhabit pelagic zones and contribute to nutrient cycling through their fecal matter

Ecological Interactions in Marine Ecosystems

• Predation shapes marine communities by controlling prey populations and influencing species' behaviors and distributions
  • Apex predators (sharks, orcas) exert top-down control on ecosystems and maintain biodiversity through trophic cascades
  • Keystone predators (sea otters) regulate the abundance and distribution of their prey (sea urchins), indirectly affecting entire ecosystems
• Competition for resources (food, space, mates) drives species adaptations and niche differentiation
• Mutualistic relationships benefit both partners, such as the symbiosis between corals and zooxanthellae or between cleaner fish and their clients
• Parasitism involves one organism (the parasite) benefiting at the expense of another (the host), potentially influencing host behavior and population dynamics
• Trophic cascades occur when changes in the abundance of a predator indirectly affect multiple lower trophic levels
• Ecosystem engineers (beavers, oysters) modify their physical environment, creating habitats for other species

Environmental Factors and Adaptations

• Temperature influences metabolic rates, species distributions, and ecosystem productivity
  • Marine organisms have evolved adaptations to cope with temperature variations (antifreeze proteins, heat shock proteins)
  • Ocean circulation patterns (currents, upwelling) affect the distribution of heat and nutrients in marine ecosystems
• Light availability decreases with depth, leading to adaptations such as enlarged eyes, bioluminescence, and vertical migration
• Salinity varies across marine environments, with organisms possessing osmoregulatory mechanisms to maintain internal balance
• Ocean acidification, caused by increased atmospheric CO2 absorption, can impair calcification in marine organisms (corals, mollusks) and alter ecosystem structure
• Nutrient availability (nitrogen, phosphorus, iron) limits primary productivity in many marine ecosystems
  • Upwelling and terrestrial runoff can enhance nutrient supply and stimulate phytoplankton growth
  • Eutrophication, caused by excessive nutrient input, can lead to algal blooms and hypoxic conditions

Human Impact and Conservation Efforts

• Overfishing has led to the decline of many commercially important fish stocks and altered marine food webs
  • Fishing practices (bottom trawling, longlines) can damage benthic habitats and cause bycatch of non-target species
  • Sustainable fishing practices (catch limits, marine protected areas) aim to balance human needs with ecosystem health
• Pollution (plastic debris, oil spills, agricultural runoff) can harm marine life through ingestion, entanglement, and habitat degradation
• Climate change impacts marine ecosystems through ocean warming, acidification, and changes in circulation patterns
  • Coral bleaching and sea level rise threaten the survival of coral reefs and associated communities
  • Shifts in species distributions and phenology can disrupt ecological interactions and ecosystem functioning
• Invasive species, introduced through human activities (ballast water, aquaculture), can outcompete native species and alter ecosystem dynamics
• Marine protected areas (MPAs) safeguard biodiversity, enhance fisheries, and promote ecosystem resilience
• Ecosystem-based management considers the complex interactions within marine ecosystems and seeks to balance human activities with conservation goals
• International agreements (UNCLOS, CBD) and organizations (IUCN, IMO) work towards the sustainable use and protection of marine resources