Lecture 26 Make sure you understand the concept of limiting factors for new productivity. Can be light in some cases, or nutrients. Macronutrients that can be limiting to marine productivity are nitrogen (in it’s “fixed” or biologically available forms, nitrate, nitrite, ammonia), phosphate, and silica (esp. for diatoms) Work in the mid-20th century by Redfield showed that the ratio of C:N:P in phytoplankton is 106:16:1. When diatoms are involved the “Redfield Ratio” becomes C:N:P:Si 106:16:1:16 Liebig’s Law of the Minimum states that phytoplankton growth is regulated not by the TOTAL amount of nutrients available but by the amount of the scarcest resource. So if plankton require 16x as much fixed N as P, and the ratio of N:P in the water is 12:1, N is the “limiting nutrient” Micronutrients can also sometimes limit productivity (Fe, Mn, Cu, Zn, B, Mo). Mostly metals. Biological requirement is very small but in some cases isn’t met and Fe for example can be the limiting nutrient. Make sure you’re familiar with sources of nutrients to marine ecosystems. N & P. Respiration is photosynthesis in reverse. Respiration by decomposers consumes oxygen, produces CO2, and “regenerates” nutrients back to seawater. The N:P ratio of seawater is typically close to 16:1 since most of the nutrients aren’t “new” (from river runoff or N-fixation locally) but are regenerated by respired organic matter. “Marine snow” is sinking organic matter. Can be sampled in the water column with a sediment trap. Lecture 27 This lecture focused on the carbon cycle and the relationship between climate, carbon, and marine productivity. Data from polar ice cores shows that atmospheric CO2 goes up during interglacial periods and down during glacial periods (like the most recent, the Last Glacial Maximum). During glacial periods, more carbon is “pumped” out of the atmosphere and into the deep ocean (below the thermocline). During interglacials, this carbon is returned to the atmosphere. We investigated two “pumps” that account for this, solubility pump and biological pump. Industrialization and fossil fuel burning has caused CO2 to increase well above levels typical of an interglacial. We can see this in Mauna Loa CO2 record, which is continuous back to 1958. As much as half of the CO2 released by human activities has been taken up by the ocean, and it’s largely in the deep North Atlantic (via North Atlantic Deep Water production). The annual cycle in atmospheric CO2 concentration reflects the balance between photosynthesis and respiration (about 1⁄2 and 1⁄2 land vs ocean). The “pumps”: “Solubility pump” is chemistry and physics. CO2 dissolves in seawater, or is released by seawater depending on temperature, salinity, pH, carbonate chemistry (the chemical part). Dissolved CO2 is carried below the thermocline by deep water formation and brought back to the surface by upwelling. That’s the physical part. Temperature matters a lot (CO2 is more soluble in cold water) so polar regions tend to be sinks (CO2 removed from atmosphere to ocean) while equatorial regions tend to be sources of CO2 from the ocean to the atmosphere. “Biological pump” is uptake of CO2 by photosynthetic phytoplankton, and then “export” of that carbon to the deep ocean as marine snow (largely as fecal pellets produced by heterotrophs that eat the phytoplankton). The strength of this “pumping” will depend largely on availability of nutrients delivered to the photic zone. Most of the sinking carbon will be regenerated by heterotrophs (largely bacteria) living below the photic zone. Only about 0.1% of sinking organic matter actually makes it into sediment for long-term storage. HNLC (high nutrient – low chlorophyll) regions are parts of the ocean where there are high concentrations of nutrients (typically fixed N) at the surface. Plankton aren’t taking them up. Why not? Iron (Fe) limitation is the general explanation for HNCL regions. Seen in places far from land where there are no local sources to meet the small biological requirement for Fe. In these places small amount of soil “dust” may be the only source of Fe, and it doesn’t amount to much. Oceanographer John Martin (1990) “Iron Fertilization Hypothesis) proposed that lower atmospheric CO2 during glacial periods reflected a stronger biological pump as Fe limitation in HNLC regions is overcome by higher delivery of Fe from atmospheric dust. Some support for this hypothesis but there are other factors at play controlling glacial CO2. Martin’s ideas spurred a lot of interest in one type of “geoengineering”. Can we add Fe to Fe-limited parts of the ocean, stimulate productivity, boost the “biological pump”, and draw down some of the CO2 added to the atmosphere by industrial activities? The answer seems to be sort of. Fe fertilization experiments seem to result in increased productivity (i.e. a short term phytoplankton bloom) but the evidence for significant “export” of carbon is weak. The plankton mostly get regenerated in the surface ocean and CO2 is released back to the atmosphere. It’s not clear this type of geoengineering would help with the atmospheric CO2 problem and it faces a lot of opposition because it could have unforeseen negative consequences. Lecture 28 We talked about Stellwagen Bank, designated in 1992 as part of the US National Marine Sanctuary program, and whale feeding ground in our backyard (Massachusetts Bay). Stellwagen Bank is an important summer feeding ground for Humpback whales due to factors from all 4 branches of oceanography (geological, physical, chemical, biological) Geological: “banks” are shallow areas in the coastal ocean. We’re on continental crust here (continental shelf). While deeper areas of the shelf can be up to 500m deep, banks are <100 meters, sometimes only 10 meters. Banks are made of sediment deposited by retreated glaciers at the end of the Last Glacial period. Glacial deposits consist of mud, sand, and gravel, and the different sediments control the benthic communities that live in them. Sand lance are a small fish that lives in sandy sediment and are an important food source for whales feeding on Stellwagen bank. Physical: Cold nutrient-rich water comes to Mass Bay from the Labrador current from the North. Enters the Gulf of Maine through one of several deep channels, circulates in small gyres. Some of the water enters Mass Bay as the Gulf of Maine Coastal Current. Tidal currents push water back and forth over banks, causing upwelling, brings nutrients to the surface. Water ultimately leaves the region to the south via another deep water channel. Increased sunlight in springtime produces a strong spring phytoplankton bloom, providing food for visiting whales. Temperate climate causes strong stratification to develop in the summer and productivity decreases as nutrients are used up. Breakdown of stratification in the fall leads to a smaller secondary bloom. Chemical: Nutrient data from Stellwagen Bank shows very low N&P in top 20-40 meters, increasing below a shallow summertime thermocline. Photic zone is only about 60 meters deep in this coastal environment. Chlorophyll maximum is around 30-40 meters where there is still light and nutrient concentrations are a bit higher than at surface. N:P ratios are ***typo on earlier version*** <16:1 indicating strong nitrogen limitation here. Biological: tagged whales can be tracked and are seen feeding in waters exhibiting the “deep scattering layer” representing high abundance of zooplankton (and small fish) over the bank feeding on phytoplankton supported by upwelling nutrients Lecture 29 This was Prof. Buston’s lecture. This was really a research talk, so there is a LOT of information here that we don’t expect you to fully digest. You’re responsible for the highlights: Coral reefs are the among the most biodiverse marine ecosystems. Biodiversity refers to the number of marine taxa (fishes, invertebrates, etc) comprising the ecosystem. High biodiversity tends to be associated with ecosystem services (recreation, food in the case of coral reefs). Loss of biodiversity due to human activities is a concern for most marine ecosystems. One approach to conservation of marine biodiversity is establishment of a global network of marine reserves (protected areas). Metapopulation ecology: a metapopulation refers to a larger population (of fishes for example) made up of many local populations. A stable metapopulation is one made up of large numbers of individuals that are well connected. Understanding the connectivity of metapopulations is important to managing biodiverse marine reserves. This is what Buston’s lab is focusing on. Old assumption on connectivity of reef fish metapopulations: larval fish (zooplankton!) are carried thousands of km by ocean currents. Recent work on connectivity shows that most fish grow up on the reefs where they were born, and that dispersal drops off to zero over distance scales around 30-40 km. These dispersal patterns are refered to as kernals, and they vary between fish species. Prof Buston described his work trying to characterize the dispersal kernal for a Belize reef fish called Goby, which live in Bikini Bottom yellow sponges. Research protocol is to catch the fish, take a clipping of fin, measure the genotype (gene sequencing) to determine relatedness, match babies to parents. Results: median dispersal of babies only 1.7km, max 15km, consistent with newer ideas on reef fish metapolulation connectivity. One implication is that the “connected” series of marine reserves along the Belizian reef aren’t really connected, at least not for all reef fish. A possible conservation solution is to establish “stepping stone” reserves between the major reserves to connect them. Lecture 30 We looked at mid-ocean ridge hydrothermal vents and the communities they support. Like the Stellwagen lecture, the idea was to look at geological, physical, chemical factors that support the biology. First mid-ocean ridge hydrothermal vents were discovered in 1977 at the Galpagos Rift by a group from WHOI using the HOV Alvin . The vents themselves were not too surprising, the group was looking for them based on several previous observations: -measurements of very hot water in the Red Sea, along with colorful, metal-rich sediments. -early ocean drilling project cores showed such metal-rich sediment was consistently found directly on top of ocean crust basalt, suggesting origin near mid-ocean ridges. -Rocks dredged and cored from the Mid-Atlantic Ridge showed evidence of hydrothermal alteration, reactions between seawater and basalt rock at temperatures way above expected for deep sea. -Ophiolites, sections of ocean crust emplaced on land by convergent tectonics also contained high concentrations of metals (iron, copper) and evidence of high temperature water-rock interaction. -measurements of heat flow from ocean floor indicated that the ocean crust was cooler than expected simply from conductive cooling. Best explained by loss of heat to seawater through seawater circulation through the hot crust. What was very surprising upon discovery of seafloor vents was the biological community they supported. Clams, worms, crabs, bacteria, all independent of energy from the sun. Vents are the exit points for seafloor hydrothermal circulation. Seawater enters fractures in the ocean crust, is heated to 350-400°C by the magma below the ridge that supports mid-ocean ridge volcanic activity, reacts chemically with the rocks making up the crust, loses oxygen, becomes acidic, and collects H2S and dissolved metals, exits the crust in vents where this hydrothermal water mixes with water that is cold, oxygenated, high pH. These huge chemical gradients supply the energy that supports the seafloor ecosystems. The black “smoke” is metal-sulfide minerals that form when hydrothermal water mixes with seawater. The whole ocean gets circulated through mid-ocean ridge hydrothermal vents on a 10-20 million year timescale. This has a big effect on seawater chemistry, perhaps as important as the effects of rivers. Chemosynthesis is a process that uses energy from reaction between chemical components that are inherently out of equilibrium (H2S from vent fluid + O2 from seawater) to produce organic matter (sugars). Compare to photosynthesis, which takes energy from the sun to drive a reaction that produces chemical components that are out of equilibrium (organic matter and O2). Either way, the result is a food source for heterotrophs. Most famous members of the hydrothermal vent community are the tubeworm Riftia. It takes in H2S and O2 from the water, transports these to its gut (the trophosome) where symbiotic chemosynthetic bacteria live. The bacteria make sugar which the worm feeds on. Vents and vent communities are transient. A variety of measures indicates that they are active only for a few decades before the vent stops emitting hydrothermal water or shifts to a different location along the ridge. What about population connectivity then? How does the community move to the new location. Some work indicates that deep sea whale carcasses can serve as a temporary home as stepping stones moving towards new active vents. Decomposing whales release a lot of H2S! Lecture 31 Coastal ocean and coral reefs. Started by revisiting a few terms and defining a few new ones. Pelagic refers to the water column as opposed to Benthic, referring to processes and organisms that live in or on the sediment at the seafloor. Oceanic province is deep water, seaward of the continental shelf. Neritic province is the shallow ocean, inclusive of the continental shelf up to the low tide line (so always underwater) Littoral province is the part of the ocean that is between the high and low tide lines, so periodically underwater or exposed to air. Also called intertidal zone. The open ocean (oceanic province) tends to be biologically sparse because nutrient concentrations are low. Most of the life in the oceanic province is in the top 200 meters (Epipelagic zone) where there is light. The coastal ocean is full of life, lots of biodiversity and lots of productivity. Diversity is a result of large gradients in light, nutrients, temperature, tides, sediment, salinity, etc. Big diversity in ecosystem/community types in coastal zone too, including coral reefs, seagrass meadows, kelp forests (all neretic), beaches, tidal flats, mangroves, salt marshes (all littoral). Estuaries are coastal zones typically at the mouths of river where freshwater and seawater mix. Chesapeake Bay and Delaware Bay are two very large estuaries on the east coast of US, and there are many smaller estuaries up and down the coast including Massachusetts. Barrier islands are sandbars that develop seaward of the coast. Lagoons are quiet water environments behind barrier islands. The islands serve as storm barriers protecting the mainland. Movement of sand absorbs storm energy. In the US and elsewhere extensive development (condos, hotels) of these vulnerable landforms is an issue. We can divide benthic environments by high energy vs low energy. Mostly dependent on water depth, does wavebase intersect bottom? But you can also find low energy environments in shallow water when protected by barriers. Benthic communities differ based on substrate: hard-bottom vs soft-bottom Muddy soft-bottom communities have more deposit feeders. Sandy soft-bottom communities have more filter feeders. Hard-bottom communities feature seaweeds, filter feeders, grazer, predators Coral reefs are high energy, neritic, hard bottom benthic communities Found in warm water, so usually between 30°N to 30°S, full salinity, clear water, shallow. Corals are sensitive to sediment, nutrient loading, acidification, temperature. Coral reefs are changing rapidly due to these pressures. 50% of tropical reef coverage lost since 1950. Most dramatic mechanism is “bleaching” where high water temperature causes corals to eject zooxanthellae (colorful symbiotic photosynthetic dinoflagellates). Solitary (non-reef building) corals are found in colder waters, including New England. These are of interest to biologists because they are more resilient, and some are capable of living either with or without zooxanthellae. Corals are animals, related to jellyfish. A coral reef is built by colonies of individual corals known as polyps. Polyps secrete calcium carbonate, building the hard reef structure over time. The calcium carbonate structure that the polyp sits in is called a corallite. The polyp has tentacles that capture food (mostly small zooplankton) and deliver it to the gut. The tentacles have stinging cells (nematocysts) as do jellyfish. Coral reefs are large platforms built up of calcium carbonate (limestone) over thousands of years. The reef-top is flat and shallow (5-10 meters). The reef face drops off into much deeper water. The reef grows upward keeping pace with rising sea level. Individual polyps lift off of their corallite bases, secrete new CaCO3, elevating the reef. There are several additional slides at the end of this lecture deck that I didn’t cover in class and won’t test you on, but worth looking at if you want a fuller picture of coral reefs, especially the videos! Lecture 32 Coastal wetlands take different forms by climate: Mangroves in the tropics, salt marshes at higher latitude. These ecosystems are highly productive and are important for storage of carbon i.e. “blue carbon” BU investigators work on both environments. Cat’s lecture focused on work on salt marshes and estuaries in New England, and on biogeochemistry, cycling of C and N. Components of a marsh are: low marsh – below normal high tide line; high marsh – above normal high tide line but flooded by spring tides, and upland – above spring tide high water line so more marsh adjacent than a part of the intertidal marsh. Ecosystem services provided by salt marshes include: improvements to water quality (largely through denitrification), recreation, flood defense, fisheries, and carbon sequestration. Coastal wetlands are hotspots for carbon storage (10x more per area than terrestrial forests) Marshes take up C (CO2 à organic matter) but also emit greenhouse gases (CO2 from respiration, N20, and methane) Methane and esp N2) have very high “global warming potential”, much more potent greenhouse effect per molecule (but much less abundant than CO2). Work by BU grad student Nia Bartolucci shows that there is net uptake of CO2 from salt marshes and emission of CH4 and N2O only offset about 1% of the greenhouse effect reduction from CO2 uptake. Excess fixed nitrogen from runoff, wastewater, fertilizer causes algal blooms and eutrophication of coastal ecosystems. Work by former BU grad student Nick Ray shows that oyster beds are effective at removing this excess fixed nitrogen via denitrification (NO3- à N2 (or N2O)). This is accomplished by denitrifying bacteria that live in association with the oysters. Cat’s thesis work will look into whether ocean acidification (from increased atmospheric CO2) will have an impact on the ability of oysters to promote denitrification. The hypothesis is that oyster health will be negatively impacted by acidification (oysters make shells of CaCO3) and reduced oyster health will decrease their ability to remove excess fixed nitrogen from coastal ecosystems. Lecture 33 Sea level varies A LOT on long timescales. 120 meters lower at Last Glacial Maximum (LGM), 20 thousand years ago. 17 meters higher than present during Pliocene, 5 million years ago. Coastline position changes radically as sea level rises/falls. Why does (eustatic) sea level change? 1) changes in the amount of water in the ocean basins, related to growth/retreat of big continental ice sheets and smaller mountain glaciers. 2) Thermal expansion. Warmer water takes up more volume. Knowledge of post – LGM sea level rise (from 120 down) comes from dating coral reef cores from Barbados and other tropical reefs. Reefs build vertically keeping pace with sea level rise. Rate was around 13 mm/year, sustained for many thousands of years, basically done by 6-8 thousand years ago, pretty stable since this time (Holocene). Higher resolution sea level records for last 2000 years come from salt marsh cores. Salt marshes also build vertically (accretion) by input of sediment and accumulating of biomass. Ideally they keep pace with sea level rise. Records show level changing very little, only +/-10cm or so over hundreds of years. Then sea level starts increasing rapidly (few mm/year) over the last ~200 years. For the last 150-200 years we can turn to tide gauge data, same gauges used to understand and make predictions of daily/monthly tide height. They show increases in sea level about 1.3 mm/yr from 1900-1970’s. However, tide gauge data need to be corrected for relative sea level change to produce a global eustatic signal. Locally you find higher or lower rates of sea level rise as the land may be either sinking (subsidence) or in some cases even rising (usually active tectonics, earthquakes). Fastest US relative sea level rise is in Mississippi Delta region where land is subsiding due to decreased sediment input (from dams on the river) and extraction of oil and gas from the seafloor. For last few decades (1992+) we can use satellite altimetry (measuring the height of the ocean from space) to monitor sea lever. These data (plus tide gauge data) indicate an increase in sea level rise to about 3.7 mm/yr over the last few decades. Why? Current sea level rise is about 50/50 melting ice and warming water. Both are due to a stronger greenhouse effect from rising atmospheric CO2 concentrations, which has accelerated in recent decades. Global mean surface temperature (land and ocean) has risen almost 1.5°C from pre-industrial baseline. Both Greenland and Antarctic ice sheets are showing consistent mass loss over the last 20 years based on GRACE data, which measures the mass of ice based on gravity from space. Lecture 34 Climate change and mitigation IPCC is a UN-sponsored international organization that writes assessment reports on the state of knowledge of climate change based on peer-reviewed published literature. The latest (2021 AR6) IPCC report indicates concerning changes underway and expected in coming decades for the ocean: increased marine heatwaves, decreasing deep water oxygen concentrations due to warming waters, continued sea level rise, continued ocean acidification. IPCC organizes modeling community around a series of “shared socio-economic pathways” that describe how humans might proceed in terms of carbon emissions in coming decades. Based on these hypothetical pathways, modelers can simulate the response of the Earth System to these various forcings. Only SSPs that involve significant net negative carbon emissions (cessation of fossil fuel burning plus carbon capture and removal) keep global warming below the +1.5°C threshold agreed upon by the Paris Agreement (2015). The most pessimistic scenarios (SSP5, which some argue is unreasonably pessimistic) predict warming of +4 to 5 °C by year 2100. Sea level rises in all of these scenarios, reaching 0.5m above 1900 baseline for SSP1, closer to 1m+ for SSP5 scenario. Which pathway we take could make a big difference for highly populated low elevation coastal cities. Looking forward a few hundred years, increasing (or even consistent) warmth should produce sea level rise in the 1-3m+ range, even for optimistic scenarios. This is going to require major changes to coastal cities for future generations to deal with. In the meantime, even modest sea level rise increases the threat of coastal flooding associated with storm surge. Boston has seen big increases in flooding even as previously deserted neighborhoods (i.e. the Seaport) have developed rapidly over the last 20 years. What are the responses? 1) Managed retreat, move infrastructure away from the coast as sea level rises and the coastline natural advances landward. Sounds great, but it’s expensive and displaces people who already live there. 2) Accommodate: build buildings and roadways that can handle regular flooding. 3) Hard protection – seawalls, barriers 4) Soft protection – maintenance of coastal dunes, salt marshes, barrier islands.
