🐠Marine Biology Unit 2 – Seawater: Physical & Chemical Properties

What's the Big Deal?

• Seawater covers approximately 71% of the Earth's surface and plays a crucial role in regulating global climate and supporting marine ecosystems
• The unique physical and chemical properties of seawater distinguish it from freshwater and significantly impact the distribution and adaptation of marine organisms
  • Salinity, temperature, and density variations create distinct layers and zones in the ocean (epipelagic, mesopelagic, bathypelagic)
• Understanding seawater properties is essential for marine biologists to comprehend the complex interactions between the ocean and its inhabitants
• Seawater's high heat capacity helps regulate Earth's temperature by absorbing and redistributing heat, moderating climate fluctuations
• The ocean's ability to absorb and store carbon dioxide (CO2) helps mitigate the effects of climate change, but excessive absorption leads to ocean acidification
• Seawater's chemical composition provides essential nutrients for primary producers (phytoplankton), forming the foundation of marine food webs
• Studying seawater properties enables scientists to predict the impacts of climate change, pollution, and other anthropogenic factors on marine ecosystems

Key Components of Seawater

• Seawater is a complex solution composed of water (H2O) and dissolved substances, primarily salts, gases, and organic compounds
• The major dissolved ions in seawater include chloride (Cl-), sodium (Na+), sulfate (SO42-), magnesium (Mg2+), calcium (Ca2+), and potassium (K+)
  • These ions contribute to the salinity and electrical conductivity of seawater
• Dissolved gases in seawater, such as oxygen (O2), carbon dioxide (CO2), and nitrogen (N2), are essential for marine life and biogeochemical cycles
  • Oxygen is crucial for respiration in marine organisms
  • Carbon dioxide is used by photosynthetic organisms (phytoplankton, algae) and influences ocean acidification
• Trace elements, such as iron (Fe), zinc (Zn), and copper (Cu), are present in small quantities but play vital roles in biological processes and limiting factors for primary production
• Organic compounds, including dissolved organic matter (DOM) and particulate organic matter (POM), contribute to the nutrient cycling and energy transfer in marine ecosystems
• The relative proportions of these components remain relatively constant in the open ocean, known as the principle of constant proportions

Salinity: More Than Just Salt

• Salinity is a measure of the total dissolved salts in seawater, expressed as parts per thousand (ppt) or practical salinity units (PSU)
  • Average ocean salinity is approximately 35 ppt or 35 PSU
• The major ions contributing to salinity are chloride (Cl-), sodium (Na+), sulfate (SO42-), magnesium (Mg2+), calcium (Ca2+), and potassium (K+)
• Salinity varies geographically due to factors such as evaporation, precipitation, river runoff, and ice formation
  • Higher salinity is found in regions with high evaporation and low precipitation (subtropical gyres)
  • Lower salinity occurs in areas with high precipitation, river inflow, or ice melt (polar regions, coastal areas)
• Vertical salinity gradients create distinct layers in the ocean, such as the halocline, which separates water masses of different salinities
• Marine organisms have evolved various adaptations to cope with salinity variations, such as osmoregulation and ion transport mechanisms
  • Osmoconformers (most invertebrates) maintain internal fluids isotonic to the surrounding seawater
  • Osmoregulators (most vertebrates) actively regulate their internal osmotic concentration
• Changes in salinity can significantly impact the distribution, behavior, and survival of marine species, as well as the functioning of marine ecosystems

Temperature and Density Dynamics

• Seawater temperature varies with depth, latitude, and season, ranging from -2°C to over 30°C
• Temperature affects the density of seawater, with colder water being denser than warmer water
  • Density is also influenced by salinity, with higher salinity increasing density
• The relationship between temperature and density creates vertical stratification in the ocean, known as the thermocline
  • The thermocline separates the warmer, less dense surface layer (epipelagic zone) from the colder, denser deep layers (mesopelagic and bathypelagic zones)
• Temperature and density gradients drive ocean circulation patterns, such as surface currents and deep water formation
  • Thermohaline circulation, or the global ocean conveyor belt, is driven by differences in temperature and salinity
• Marine organisms have adapted to specific temperature ranges, with many species exhibiting temperature-dependent distribution patterns
  • Coral reefs thrive in warm, shallow waters (25-29°C) and are sensitive to temperature fluctuations
  • Polar species, such as penguins and polar bears, are adapted to cold temperatures and rely on sea ice habitats
• Climate change-induced warming of the oceans can lead to coral bleaching, species migrations, and alterations in marine ecosystem structure and function

Light and Sound in the Ocean

• Light penetration in seawater depends on wavelength, with shorter wavelengths (blue light) penetrating deeper than longer wavelengths (red light)
  • The photic zone, where sufficient light penetrates for photosynthesis, extends to approximately 200 meters depth
  • Below the photic zone, the aphotic zone receives no sunlight and is perpetually dark
• Light attenuation in seawater is influenced by factors such as dissolved substances, suspended particles, and plankton abundance
  • Coastal waters often have higher turbidity and lower light penetration compared to open ocean waters
• Marine organisms have evolved various adaptations to optimize light capture or navigate in low-light environments
  • Photosynthetic organisms (phytoplankton, algae) have pigments that absorb specific wavelengths of light
  • Deep-sea organisms may possess bioluminescent organs or enlarged eyes to detect faint light signals
• Sound travels approximately five times faster in seawater than in air and can propagate over long distances
  • The speed of sound in seawater is influenced by temperature, salinity, and pressure
• Many marine organisms rely on sound for communication, navigation, and prey detection
  • Whales and dolphins use echolocation to navigate and locate prey in the dark ocean depths
  • Snapping shrimp produce loud snapping sounds that contribute to the underwater soundscape
• Anthropogenic noise pollution, such as shipping and seismic surveys, can disrupt marine animal behavior and communication

Chemical Reactions Underwater

• Seawater's unique chemical composition and properties influence the types and rates of chemical reactions that occur in the ocean
• The pH of seawater is typically slightly alkaline, ranging from 7.8 to 8.2, due to the buffering capacity of dissolved ions
  • Ocean acidification, caused by increased absorption of atmospheric CO2, can lower seawater pH and impact marine organisms
• Dissolution and precipitation reactions are common in seawater, affecting the cycling of elements and the formation of geological structures
  • Calcium carbonate (CaCO3) precipitation is essential for the formation of coral reefs and shells of marine organisms
  • Silica (SiO2) dissolution and precipitation are important for the growth of diatoms and the formation of siliceous oozes
• Redox reactions, involving the transfer of electrons, play a crucial role in biogeochemical cycles and the metabolism of marine microorganisms
  • Photosynthesis and respiration are essential redox reactions that drive the carbon cycle in the ocean
  • Sulfate reduction and methane oxidation are important redox processes in marine sediments
• Adsorption and desorption reactions between dissolved substances and particles influence the transport and availability of nutrients and pollutants in seawater
  • Iron adsorption onto sinking particles can limit primary production in some regions of the ocean
  • Adsorption of pollutants onto microplastics can facilitate their entry into marine food webs

How These Properties Shape Marine Life

• The physical and chemical properties of seawater create distinct habitats and niches that shape the distribution and adaptation of marine organisms
• Salinity gradients influence the osmotic balance and ion regulation strategies of marine species
  • Estuarine organisms, such as oysters and crabs, have evolved to tolerate wide salinity fluctuations
  • Freshwater influx from rivers creates brackish habitats that support unique communities of organisms
• Temperature variations affect metabolic rates, reproductive cycles, and species distributions
  • Coral reefs are confined to warm, shallow waters due to the temperature sensitivity of coral-algal symbiosis
  • Polar regions support cold-adapted species, such as seals and penguins, which rely on insulating fat layers and specialized hunting strategies
• Light availability determines the vertical distribution of photosynthetic organisms and the structure of marine food webs
  • Phytoplankton communities are most abundant in the well-lit surface layers of the ocean
  • Deep-sea organisms have evolved adaptations to cope with the absence of sunlight, such as bioluminescence and specialized sensory organs
• Chemical gradients, such as oxygen and nutrient concentrations, influence the distribution and behavior of marine organisms
  • Oxygen minimum zones (OMZs) host unique microbial communities adapted to low-oxygen conditions
  • Nutrient upwelling regions support high primary productivity and rich marine biodiversity
• The interplay between physical and chemical properties creates complex and dynamic marine ecosystems that support a wide range of life forms and ecological interactions

Real-World Applications and Research

• Understanding seawater properties is crucial for predicting and mitigating the impacts of climate change on marine ecosystems
  • Monitoring ocean temperature, salinity, and pH helps track the progression of climate change and its effects on marine life
  • Modeling ocean circulation patterns and heat transport is essential for projecting future climate scenarios and sea level rise
• Seawater analysis is used in environmental monitoring and pollution assessment
  • Measuring concentrations of pollutants, such as heavy metals and persistent organic pollutants (POPs), helps identify sources and assess ecological risks
  • Tracking the spread of oil spills and plastic debris relies on understanding ocean currents and density gradients
• Marine biotechnology and bioprospecting benefit from knowledge of seawater's chemical composition and the adaptations of marine organisms
  • Screening marine microorganisms for novel compounds and enzymes can lead to the development of new pharmaceuticals and industrial applications
  • Studying the biomineralization processes of marine organisms inspires the development of advanced materials and technologies
• Fisheries management and aquaculture practices are informed by seawater properties and their influence on marine species
  • Identifying optimal temperature and salinity ranges for cultured species helps maximize productivity and minimize stress
  • Monitoring dissolved oxygen levels and nutrient concentrations is crucial for maintaining the health of aquaculture systems
• Ocean exploration and mapping rely on understanding the physical and chemical properties of seawater
  • Acoustic mapping techniques, such as sonar and multibeam echosounders, utilize the properties of sound propagation in seawater
  • Autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) are designed to withstand the pressure and corrosive nature of seawater