| Ocean Physics | Wind driven - Surface currents
Density driven - Thermohaline circulation
Gravity driven - Tides |
| Wind driven surface currents | Driven by atmospheric circulation. Uneven warming of Earth’s surface. Warming near the equator leads to air to rise, leaving a local low pressure (LP). LP sucks in northerly / southerly waters: - vertical Hadley cells, - In turn drive ferrel cells. Add in a spinning sphere: -Coriolis effect, -right in N. hemisphere, -left in S. hemisphere. |
| Horizontal water movements | Coriolis effects: Equatorial parcel of water/air is moving faster than increased latitude and it leads to a deflection in absolute vector in N-S flow. E-W current flow blocked by continents. Water piles up: Counter-currents and N-S movements. Gyres |
| W vs E boundary currents | West currents: ~ 200 cm/s, narrow, deep, polewards flow, bringing warm water e.g. Gulf Stream, Kuroshio.
East currents: < 20 cm/s, broad, shallow, equatorwards flow, bringing cold water e.g. Canaries, California |
| Coriolis effect on depth | Surface waters moved by wind (Rt/NH, L/SH). As water at surface pushes on water below, this
water moves yet further to the R (or L) of wind etc. Movement progressively weaker with depth. At 90-100 m depth: velocity is zero with direction of 180 degrees from wind. Net water movement (all depths) ~ 90 degrees from wind. |
| Ekman at Boundary Coasts | Coastal upwelling. Winds blow surface waters away from coast (nb: Coriolis). Sub-surface, cold, nutrient-rich waters come to surface. High productivity: Important for whole ecosystem
productivity and fisheries. |
| Thermohaline circulation | Global scale Ocean circulation involving surface and deep Ocean. Driven by changes in density (fluid dynamics), and interactions with land mass and seabed topography. Density is a function of temperature and salinity. Extremely important for sequestering heat from surface Oceans. Geostrophic flows derive from thermohaline circulation. |
| Mesoscale eddies | Thermohaline circulation leads to mesoscale processes. When we have two opposing geostrophic flows, their interaction breaks down into turbulent flow. |
| How does Ocean physics control Ocean biology? | Temperature, sunlight and seawater density. Through biogeochemistry: The carriage of nutrients. |
| Sverdrup’s critical depth hypothesis | 1. photosynthesis capacity: two lines indicating the compensation (growth rate =0) and critical depth (photosynthesis = respiration). 2. Deep mixing: In winter, phytoplankton spend more time below the compensation depth. In spring, Phytoplankton on average spend more time above the compensation depth. |
| Limitations of Sverdrup’s critical depth hypothesis | 1. Winter mixing is thorough to mix phytoplankton evenly throughout the water column
2. Loss rate of phytoplankton (grazing, viral lysis, cell death) independent of depth or growth rate
3. Nutrients do not limit instantaneous growth of phytoplankton |
| Blooming | Blooms terminate in summer. Nutrients are still available. Grazing is important and growth rate is the highest in the winder (Why?) |
| What is biodiversity? | Variety of different species, genetic variability of individuals within a species and variety of ecosystems. At the core of ecology is the ability to count things! 3 Major levels of biodiversity: Genetic diversity, Species diversity and Ecosystem diversity. |
| Biodiversity assessment | Problem: The data is multivariate (abundance many variables/species measured).
Solution: Turn your data univariate via a diversity metric |
| Taxonomic measures | Identity of species matters. Species identity unimportant with conventional measures. Same value of diversity coefficient: two different assemblages, contrast with taxonomic measures. Progressively greater weighting factor to more distantly related species. Possible to have low species richness, but high taxonomic distinctness. Relatively independent of sampling intensity.
Some evidence of sensitivity to environmental disturbance. Calculation manually for small data sets. |
| Multivariate assessment | Useful when comparing between multiple sites e.g. when separated by space or time. Takes into account the exact species composition |
| Distance metrics | Aim: To reduce the “complexity” of the dataset by finding the “distance” between sites. Species count data often contains zeros and is not normally distributed. Traditional distance metrics cannot be used. Bray-Curtis metric often employed. |
| Ordination of sites in two dimensions | Ordination onto a Cartesian space that preserved the actual distance between sites (given my the distance metric). Randomly assign points in space Calculate geometric distances in the Cartesian space Compare to actual distances derived from distance metric Reassign points and optimise. |
| Other biodiversity measures | Indicator species, Endemics/rarity, Remote sensing |
| Remote sensing | The use of electromagnetic radiation to acquire information about the ocean without being in physical contact with the object, surface or phenomenon under investigation. |
| Four main types of remote sensing | Satellite, LIDAR, RADAR, Drone |
| Advantages and disadvantages of remote sensing | Advantages: 1. Scale 2. Cost. Disadvantages: 1. We can only observe what happens on the surface 2. Obscured viewing (clouds). |
| Satellite Orbits | Three main types: Sun-synchronous (Retrograde, Altitude of 800 km, Period of ~90 mins), Low-inclination (Orbit of 350 km, Inclination of 35°) and Geosynchronous orbit (35.8k km above equator, 23.93 hr period). |
| Instrumentation | Active: Altimeters, Scatterometers. Passive: Microwave radiometer, Infrared radiometer and VIS |
| mote sensing of physical phenomena | Winds and currents, Sea surface height, SST and Sea Ice |
| Scatterometers (Active) | Surface roughness correlated with wind. Manifests as increase in scattering of a microwave pulse. Principle: Multiple peaks at the same field of View either by different directions or polarisations. Wind speed negatively proportional to amount of scattering. Wind direction function of azimuthal direction between of wind and scatterometer look angle. Two looks means we can calculate direction. |
| Sea Surface Height (SSH) | We can measure the heights of the Ocean surface with a satellite mounted altimeter. The radar emits short pulses of energy vertically downward and receives the reflected signal. A vital tool for documenting and predicting sealevel change. Sea-level change driven by two phenomena: Melting of glaciers and land ice sheets, Thermal expansion. SSH must be given relative to the reference geoid. hs = H – h |
| Sea Surface Temperature | Vital to document and predict effects of climate change. Predicting the intensity of extreme weather events (typhoons and hurricanes, Bermuda Atlantic Time Series and reinsurance). Understanding the regional extent of Ocean biology. Global climate change, regional support of fisheries, ship routing, numerical weather prediction |
| GHRSST | Produce a standardised measure of SST from different sources. Does this through an ensemble algorithm. Combines passive IR and microwave radiometer sensors aboard 6 sun synchronous satellites, 4 geosynchronous satellites and thousands of in situ calibration measures. |
| Remote sensing of primary productivity | Oxygenic photosynthesis is the main process by which energy and matter enters the surface Ocean. A diagnostic pigment of oxygenic photosynthesis is chlorophyll a. The amount of chlorophyll a is proportional to the photosynthetic biomass and related to the amount of primary productivity |
| Global ocean carbon budget | The problem: Ocean colour sensing measures chlorophyll a biomass, we need NPP, which is a rate measurement. Marine phytoplankton : Only make 25% of the vegetation biomass. But the account for up to 50% of Earth’s NPP. Entire phytoplankton biomass must be turned over every 2-6 days. |
| Phytoplankton functional types | Phytoplankton can be split into ‘functional’ types: Diatoms (silicified frustules) – large and dense, Coccolithophores – CaCO3 liths (OA sensitive), Chlorophytes, Cyanobacteria - small |
| Applications of remote sensing | Data have to be ground trothed to some degree. Important habitat as a nursery ground for commercial fishes as well as a home to protected species. Seagrass mapping: can be mapped through analysis of optical imagery, subtidal habitat maps. Coral Reefs. Fishing vessel monitoring.: blue belt programme project (BBPP). |
| Intertidal systems | Adaptions for both wet and dry conditions. Continuous movement of sediment and nutrients. Habitats for grazing and filtering organisms eg. urchins, sea anemones, barnacles etc. |
| Sediments and mudflats | Sheltered areas (bays, lagoons, and estuaries). Deposition of estuarine silts, clays and organic matter. Apart from deposition there can be a complex stratified microbial community (including algae and cyanobacteria), hence, primary production may occur |
| Salt marshes | Salt-tolerant plants such as herbs, grasses, or low shrubs Plants are terrestrial in origin Trap and bind sediments providing coastal protection. Typically include sheltered environments such as embankments, estuaries and the leeward side of barrier islands and spits, or even lagoons |
| Mangroves | Ecosystems: high organic matter concentrations, high salinity variations (0.5-8%), trap and bind sediments providing, coastal protection, habitat for a large variety of species. Mangroves are Halophyte trees. Root and leaf adaptations: Salt filtration system, Wave action, Anaerobic soils |
| Seagrass meadows | Biology: Plants are terrestrial in origin, Angiosperms, Grow in marine, fully saline environment, Grow by rhizome extension or via seed. Ecology: They create their own habitat, Leaves slow down current and increase sedimentation, Roots stabilize the seabed, reduce coastal erosion, High primary production, Habitat and shelter for fish. |
| Kelp forests | Large seaweeds (algae) belonging to the brown algae (Phaeophyceae) in the order Laminariales. Similar ecological characteristics as seagrass meadows but much higher primary productivity. High growth rates (0.5 meters/day) up to 30-80m tall. Growth occurs at the base of the meristem, where the blades and stipe meet. Require nutrient-rich water with temperatures between 6 and 14 °C |
| Coral reefs | Biology: Corals are made by polyps belonging to a group of animals known as Cnidaria, They secrete calcium carbonate and build hard carbonate exoskeletons that support and protect the
coral polyps, Feeding via filtration and photosynthesis. Ecology: High productivity in marine waters that contain low nutrients, Reefs grow best in warm, shallow, clear, sunny and agitated waters, Shoreline protection, Habitat for an extremely high biodiversity |
| Upwelling systems | Motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. Nutrients + light = phytoplankton bloom. High primary productivity, rich in life. Upwelling areas are coastal (can be seasonal), equatorial
and southern ocean. |
| Seasonal blooming systems | They come as a consequence of upwelling systems or, simply, after dark/cold periods when a build-up of inorganic nutrients occurs. A large phytoplankon bloom takes place when these systems are irradiated by the sun (energy for photosynthetic organisms and increase
in temperature). High primary productivity. |
| Ocean (subtropical) gyres | Major oceanic systems. Large system of rotating ocean currents. Gyres are caused by the Coriolis effect. Extremely oligotrophic systems (nutrient-poor). Low primary production although, due to their vast extension, they play a key role at a global scale. Systems dominated by highly adapted microorganisms. |
| Deep sea systems | Mesopelagic zone (200-1k m): Light is faint, not enough to support photosynthesis. Animals at this depth include octopus, squid and hatchet fish. Bathypelagic zone (1-4km): No light penetrates this deep, so it is extremely cold and completely dark. About 1% of all ocean species lives in this zone. Abyssopelagic zone (4-6km): Water temperatures border on freezing (pressure avoids ice formation). Very few creatures live this deep. Hadalpelagic zone (6-10km): Enormous pressure of around 1.1 tonnes per square centimetre. |
| Deep sea ocean floor | Dark benthic zone. The majority of the ocean floor lies upon the ocean crust (4000m to 6000m deep). As in the water column, life is sustained by sinking organic matter. Filtering and grazing animals |
| Hydrothermal vents | Only productive systems in dark ocean. Life is based on chemosynthetic microorganisms (oxidize reduced minerals, mainly hydrogen sulfide). Fascinating symbiosis with larger organisms. Non-stable (vent-dependent). |
