Marine biology is the branch of biology concerned with organisms that live in marine environments and the biological processes occurring in oceans, seas, estuaries and related coastal systems. It encompasses the study of marine microorganisms, algae, plants, invertebrates, fish, reptiles, birds and marine mammals, as well as their physiology, behaviour, reproduction, genetics, evolution, ecology and interactions with their environments. Marine biology is closely connected with oceanography, ecology, zoology, microbiology, chemistry, geology and climate science.
The marine environment is extremely diverse, ranging from sunlit shallow coastal waters to permanently dark deep ocean habitats, hydrothermal vents, polar seas, coral reefs, kelp forests, seagrass meadows, mangroves and open ocean ecosystems. Marine biologists therefore investigate organisms adapted to very different temperatures, pressures, salinities, oxygen concentrations, light levels and nutrient conditions.
Marine ecosystems are shaped by physical and chemical conditions that differ substantially across space and depth. Salinity affects osmoregulation, temperature influences metabolism and reproduction, pressure increases dramatically with depth, and light declines rapidly beneath the surface. Nutrient availability strongly influences biological productivity, particularly in regions where upwelling brings nutrient rich deep water towards the surface. Ocean currents transport heat, nutrients, larvae and organisms over large distances and can connect geographically separated populations.
Tides and waves influence coastal habitats, while dissolved oxygen concentrations can determine which organisms can survive in particular regions. Marine biology therefore requires understanding organisms together with their physical environment. Measurements of temperature, salinity, depth, dissolved gases, nutrients and currents are frequently combined with biological observations to explain patterns of marine life.
Marine biodiversity includes enormous numbers of microorganisms and larger organisms occupying specialised ecological niches. Phytoplankton perform photosynthesis and form the base of many marine food webs, while zooplankton consume phytoplankton and other organisms. Fish, molluscs, crustaceans, echinoderms, cnidarians, annelids and numerous other invertebrate groups occupy different ecological roles. Marine mammals such as whales, dolphins, seals and sirenians have evolved specialised adaptations for aquatic life, while seabirds and reptiles depend on marine ecosystems to varying degrees. Benthic organisms live on or within the seabed, whereas pelagic organisms live in the water column. Plankton generally drift with currents, while nekton can actively swim against them. These ecological categories describe lifestyles and environments rather than rigid taxonomic groups.
Marine physiology examines how organisms function under aquatic conditions. Fish and many invertebrates must regulate internal water and ion concentrations despite differences between their body fluids and surrounding seawater. Gills provide surfaces for gas exchange and can also participate in ion regulation. Marine mammals have specialised adaptations for diving, including mechanisms that help manage oxygen stores and pressure changes, while many deep sea organisms possess biochemical and structural adaptations to darkness and high pressure.
Photosynthetic organisms must obtain sufficient light and nutrients, and some deep ocean communities depend on chemosynthetic microorganisms rather than sunlight. Reproductive strategies vary widely, with species releasing enormous numbers of eggs and larvae, producing relatively few offspring with extensive parental care, or using other strategies suited to particular environments.
Marine ecology examines relationships among organisms and their environments. Predator and prey interactions, competition, mutualism, parasitism, grazing and decomposition influence marine communities. Coral reefs illustrate complex ecological networks involving corals, photosynthetic symbionts, fish, invertebrates, microbes and physical habitat structures. Kelp forests provide habitat for numerous organisms and are influenced by herbivory, nutrient conditions and predator populations. Estuaries combine freshwater and marine influences and can be highly productive nursery environments for fish and invertebrates. Mangrove forests protect coastlines, store carbon and provide habitat for diverse organisms. Food webs can span enormous distances, and energy and nutrients move between surface waters, deeper waters and sediments. Marine biology therefore considers both individual species and ecosystem level processes.
Marine organisms have substantial importance for global biogeochemical cycles. Phytoplankton contribute significantly to global photosynthesis and carbon fixation, while microbial communities recycle nutrients and organic matter. The biological carbon pump transfers some carbon from surface waters into deeper ocean environments when organic material sinks or is transported downward. Marine sediments can store organic carbon and preserve chemical records of past environmental conditions. Nitrogen and phosphorus cycling involve numerous microbial processes that influence ocean productivity.
Because marine ecosystems interact with the atmosphere, land and ice, changes in ocean biology can have effects beyond the immediate marine environment. Conversely, changes in temperature, ocean chemistry, nutrient availability and circulation can alter marine biological communities. Marine biology consequently contributes to understanding the Earth system as a whole.
Human activities are important subjects in contemporary marine biology. Fishing can alter population sizes, age structures, food webs and community composition, while destructive practices can damage habitats. Pollution can introduce plastics, nutrients, chemicals, pathogens and other substances into marine environments. Excess nutrient input can promote eutrophication and oxygen depletion in some coastal waters. Rising ocean temperatures can shift species distributions and affect physiological processes, while increasing carbon dioxide concentrations contribute to ocean acidification through changes in seawater carbonate chemistry. Habitat destruction affects coral reefs, mangroves, seagrasses, estuaries and coastal wetlands. Marine conservation uses protected areas, fisheries management, habitat restoration, monitoring programmes and other approaches to reduce ecological damage, although effectiveness depends on local conditions, governance and ecological processes.
Marine biology uses field observation, specimen collection, microscopy, molecular analysis, acoustic technologies, satellite observations, autonomous vehicles, remotely operated vehicles and long term ecological monitoring. Environmental DNA can help detect organisms from genetic material present in water or sediment, while tagging can provide information about animal movement and migration. Marine research is challenging because much of the ocean is difficult to access, conditions can be hazardous, and organisms can be widely dispersed or highly mobile.
Nevertheless, technological developments increasingly permit detailed study of deep sea and remote environments. Marine biology is not simply the study of fish or ocean animals. It is a broad biological science concerned with life and ecological processes in marine systems, integrating molecular mechanisms, organismal adaptations, population dynamics, communities, ecosystems and global environmental change.
