Tag: dead zones

Global Biodiversity and Ecosystem Services, Marine Ecosystems

Marine Ecosystems

Marine ecosystems, from coastal to deep sea, now show the influence of human actions, with coastal marine ecosystems showing both large historical losses of extent and condition as well as rapid ongoing declines (established but incomplete) {2.2.5.2.1, 2.2.7.15}

  • Direct exploitation of fish and seafood has the largest relative impact in the oceans (well established) {2.2.6.2}.
  • The direct driver with the second highest impact on the oceans is the many changes in the uses of the sea and coast land (well established) {2.2.6.2} (See Summary of Observed Changes)

Distribution Of Changes In The Global Ocean

Source:  Halpern  (B. S., Walbridge, S., Selkoe, K. A., Kappel, C. V., Micheli, F., D’Agrosa, C., … Watson, R. (2008). A Global Map of Human Impact on Marine Ecosystems. Science, 319(5865), 948–952.doi:10.1126/science.1149345 

1.  Every square kilometer of the ocean is affected by some anthropogenic driver of ecological change

2.  Over 41% of the world’s oceans have medium to very high cumulative impacts:

  • North and Norwegian seas, South and East China seas, Eastern Caribbean, North American eastern seaboard, Mediterranean, Persian Gulf, Bering Sea, and the waters around Sri Lanka.

3.  Lowest impact areas include high-latitude Arctic and Antarctic poles, but this may change due to future polar ice loss

4.  Ecosystems with highest impacts include hard and soft continental shelves and rocky reefs, and coral reefs.

Summary of Observed Changes

Temperature Changes

  •  Rising sea surface temperatures; reduction in oxygen content; stratification; coral bleaching
  • Heat content increasing deeper into the ocean
  • Global ocean animal biomass consistently declines with climate change, and that these impacts are amplified at higher trophic levels (Source: Lotze, et al., Global ensemble projections reveal trophic amplification of ocean biomass declines with climate change. http://www.pnas.org/cgi/doi/10.1073/pnas.1900194116) Ocean biomass declines with climate change
Source: Ocean biomass declines with climate change

Rising Sea Levels (“Looking past the horizon of 2100” Nature Climate Change)

  • Thermal expansion (warmer water expands)
  • Increased runoff from land-based ice sheets
    • Greenland
    • Antarctica
    • Mountain Glaciers

Changes in the Salinity of Seawater

  • Changes to the thermohaline circulation
  • Increased likelihood of seawater stratification

Threats to Coastal Populations and Infrastructure

  • Salt-water intrusion
  • Drowning forests
  • Inland migration of wetlands

Arctic Sea Ice is Shrinking

  • Warming air temperatures increase rate of melting
  • Positive feedback albedo loop

Increasing Ocean Acidity

  • Absorption of CO2 from atmosphere
  • Impacts to shell-forming species, including coralsAbsorption of CO2 from atmosphere

Land-Based Impacts

1.  Inorganic and organic pollutants

2.  Nutrient runoff (point and non-point) and associated hypoxia

Gulf of Mexico Dead Zone

3.  Aquaculture

4.  Invasive species

5.  Habitat destruction and alteration; e.g., coastal engineering; tourist development;  salt marshes, mangroves, seagrass beds, coral reefs

6.  Input and transfer of waterborne pathogens (runoff, aquaculture)

7.  Air pollution

a.  Toxic chemicals; e.g., POP’s over the Arctic zone by airborne volatilization;

b.    Mercury                          

Source:  Lavoie, R.A., Bouffard, A., Maranger, R., Amyot, M., 2018. Mercury transport and human exposure from global marine fisheries. Sci. Rep. 8, 6705.

Significance:  Human activities have increased the global circulation of mercury, a potent neurotoxin. Mercury can be converted into methylmercury, which biomagnifies along aquatic food chains and leads to high exposure in fish-eating populations.

Trends:  Mercury export from the ocean increased over time as a function of fishing pressure, especially on upper-trophic-level organisms. In 2014, over 13 metric tonnes of mercury were exported from the ocean. Asian countries were important contributors of mercury export in the last decades and the western Pacific Ocean was identified as the main source.

Estimates of per capita mercury exposure through fish consumption showed that populations in 38% of the 175 countries assessed, mainly insular and developing nations, were exposed to doses of methylmercury above governmental thresholds.

Given the high mercury intake through seafood consumption observed in several understudied yet vulnerable coastal communities, we recommend a comprehensive assessment of the health exposure risk of those populations.


Temporal trends (1950‒2014) of marine fisheries catches (dashed lines) and mercury exported (full lines). (a) Entire ocean (blue) and coastal (red) and high seas (orange) with breakpoint (circle) ± standard error (horizontal line). (b) Entire ocean (blue) and trophic levels (TL) rounded to the nearest integer: 4 (red), 3 (orange), and 2 (green).

Nutrients (N,P): Eutrophication, Dead Zones

Plastics (shipping, fishing, land-based sources)

  • Microplastics (nanoparticles; food chain)
  • Macroplastics (ingestion, entanglement, habitat loss (shorelines))
    • Great Pacific Garbage Patch

Source:   Lebreton, L, Slat, B, Ferrari, F, Sainte-Rose, B, Aitken, J, Marthouse, R, Hajbane, S, Cunsolo, S, Schwarz, A, Levivier, A, Noble, K, Debeljak, P, Maral, H, Schoeneich-Argent, R, Brambini, R & Reisser, J 2018, ‘Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic‘, Scientific Reports, vol. 8, no. 4666. https://doi.org/10.1038/s41598-018-22939-w

  • Ocean plastic can persist in sea surface waters, eventually accumulating in remote areas of the world’s oceans.
  • Our model, calibrated with data from multi-vessel and aircraft surveys, predicted at least 79 (45–129) thousand tonnes of ocean plastic are floating inside an area of 1.6 million km2; a figure four to sixteen times higher than previously reported.
  • Over three-quarters of the GPGP mass was carried by debris larger than 5 cm and at least 46% was comprised of fishing nets. Microplastics accounted for 8% of the total mass but 94% of the estimated 1.8 (1.1–3.6) trillion pieces floating in the area.
  • Plastic collected during our study has specific characteristics such as small surface-to-volume ratio, indicating that only certain types of debris have the capacity to persist and accumulate at the surface of the GPGP.

Finally, our results suggest that ocean plastic pollution within the GPGP is increasing exponentially and at a faster rate  than the surrounding waters.


Decadal evolution of microplastic concentration in the GPGP. Mean (circles) and standard error (whiskers) of microplastic mass concentrations measured by surface net tows conducted in different decades, within (light blue) and around (dark grey) the GPGP. Dashed lines are exponential fits to the averages expressed in g km−2: f(x) = exp(a*x) + b, with x expressed in number of years after 1900, a = 0.06121, b = 151.3, R2 = 0.92 for within GPGP and a = 0.04903, b = −7.138, R2 = 0.78 for around the GPGP.

Land Loss in Coastal Areas

Source:  Mentaschi, et al., Global long-term observations of coastal erosion and accretion, Scientific Reports volume 8, Article number: 12876 (2018) https://www.nature.com/articles/s41598-018-30904-w

On a global scale, between 1984 and 2015, the loss of permanent land in coastal areas amounts to almost 28,000 km2, roughly equivalent to the surface area of Haiti (Fig. a). This is almost twice as large as the surface of gained land (about 14,000 km2) over the same period. On the other hand, the overall surface of gained active zone (about 25,000 km2) is more than two times larger than the surface of lost active zone (about 11,500 km2). Overall, the gain of active zone roughly balances the loss of land, and the gain of land balances the loss of active zone. This translates into a net loss of approximately 14,000 km2 of surface for human settlements and terrestrial ecosystems (Fig. b).


Source:  Mentaschi, et al., Global long-term observations of coastal erosion and accretion, Scientific Reports volume 8, Article number: 12876 (2018)
 
On a global scale, between 1984 and 2015, the loss of permanent land in coastal areas amounts to almost 28,000 km2, roughly equivalent to the surface area of Haiti (Fig. a). This is almost twice as large as the surface of gained land (about 14,000 km2) over the same period. On the other hand, the overall surface of gained active zone (about 25,000 km2) is more than two times larger than the surface of lost active zone (about 11,500 km2). Overall, the gain of active zone roughly balances the loss of land, and the gain of land balances the loss of active zone. This translates into a net loss of approximately 14,000 km2 of surface for human settlements and terrestrial ecosystems (Fig. b).

Size distribution of cross-shore transition length above 50 m, for erosion and accretion of land (a) and active zone (b). Lost (c) and gained (d) land around the world; lost (f) and gained (g) active zone; balance gained – lost land (e) and active zone (h). Maps show the length of cross-shore erosion and accretion aggregated on coastal segments of 100 km. In all the maps the 4 spots with the highest local transition along a 250 m transect are indicated.
 

Drivers of shoreline change around the world

List of local cases of erosion/accretion discussed in Mentaschi, et al https://www.nature.com/articles/s41598-018-30904-w The legend provides for each spot a brief summary of the drivers of shoreline change
Land Loss in Coastal Areas: Shoreline modification “masks” true rates of shoreline change along the Atlantic Seaboard

Source:  Armstrong, S. B., &  Lazarus, E. D. ( 2019).  Masked shoreline erosion at large spatial scales as a collective effect of beach nourishment. Earth’s Future,  7,  74– 84. https://doi.org/10.1029/2018EF001070

 (R)ecent rates of shoreline change along the U.S. Atlantic Coast are, on average, less erosional than historical rates. This shift has occurred despite evidence of intensified environmental forcing, including acceleration in rates of relative sea‐level rise and increased significant wave height in offshore wave climates. We suggest that the use, since the 1960s, of beach nourishment as the predominant form of mitigation against chronic coastal erosion in the United States…could explain the unexpected reversal in shoreline‐change trends.

Using U.S. Geological Survey shoreline records from 1830–2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of −55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea‐level rise over the same period.

Cumulative sediment input from decades of beach‐nourishment projects may have sufficiently altered shoreline position to mask “true” rates of shoreline change. Our analysis suggests that long‐term rates of shoreline change typically used to assess coastal hazard may be systematically underestimated. We also suggest that the overall effect of beach nourishment along of the U.S. Atlantic Coast is extensive enough to constitute a quantitative signature of coastal geoengineering and may serve as a bellwether for nourishment‐dominated shorelines elsewhere in the world.

Biogeochemical Flows, Oxygen Minimum Zones

Oxygen Minimum Zones

Declining Oxygen in the World’s Ocean and Coastal Waters

Source for the following material unless otherwise noted:   Breitburg, D., Grégoire, M. and Isensee, K. (eds.). Global Ocean Oxygen Network 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp.  The Ocean is Losing its Breath

Significance:

  1. Insufficient oxygen reduces growth, increases disease, alters behaviour and increases mortality of marine animals, including finfish and shellfish. The quality and quantity of habitat for economically and ecologically important species is reduced as oxygen declines.
  2. Finfish and crustacean aquaculture can be particularly susceptible to deoxygenation because animals are constrained in nets or other structures and cannot escape to highly-oxygenated water masses.
  3. Deoxygenation affects marine biogeochemical cycles; phosphorus availability, hydrogen sulphide production and micronutrients are affected.
  4. Deoxygenation may also contribute to climate change through its effects on the nitrogen cycle. When oxygen is insufficient for aerobic respiration, microbes conduct denitrification to obtain energy. This produces N2O – a powerful greenhouse gas – as well as N, which is inert and makes up most of the earth’s atmosphere.

Impacts of Excess Nutrients (eutrophication) on ocean oxygen

Nutrients – primarily nitrogen and phosphorus – from human waste, agriculture and industry, fertilize coastal waters. In a process called eutrophication, these nutrients stimulate photosynthesis, which increases the growth of algae and other organisms (See figure below). This results in more organic material sinking into deep water and to the sediment. Increased respiration by animals and many microbes eating or decomposing this organic material uses oxygen. The consequence can be oxygen concentrations that are far lower than those that would occur without human influence, and in some cases a complete lack of oxygen in bottom waters. Strong density differences between surface and bottom waters (referred to as ‘stratification’), due to temperature and salinity, can isolate bottom waters from the atmosphere and reduce or prevent re- oxygenation through ventilation. Semi-enclosed seas (e.g. the Black and Baltic Sea) can be sensitive to eutrophication and related deoxygenation because of their characteristic limited water exchange with the open ocean, and low ventilation rates.

Eutrophication:  nutrient flux of nitrogen and phosphorus from human waste, agriculture and industry, fertilize coastal waters


Impacts of excess nutrients (eutrophication) on ocean oxygen. (Figure modified from https://upload.wikimedia.org/ wikipedia/commons/d/dd/Scheme_eutrophication-en.svg)

Deoxygenation Facts

Oxygen Minimum Zones (OMZs)

OMZs are places in the world ocean where oxygen saturation in the water column is at its lowest, are shown in blue.  Areas with coastal hypoxia are shown in red.   Hypoxic conditions are often defined as 2 mg/L O2.

The number of water bodies in which hypoxia associated with eutrophication has been reported has increased exponentially since the 1960s; hundreds of systems worldwide have been reported with oxygen concentrations <2 mg L-1, lasting from hours to years. The increasing severity and prevalence of this problem reflects the invention and increasing use of synthetic fertilizers and the growing human population. Global fertilizer use shown includes data of fertilizers with nitrogen, phosphate and potash.

World’s two largest dead zones

Source:  European Environment Agency Ocean Oxygen Content Indicator Assessment 


Distribution of oxygen-depleted ‘dead zones’ in European seas

Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion has also been observed in other European seas in recent decades.

The Gulf of  Mexico dead zone is the world’s second largest in the world

Source:  EPA Gulf of Mexico Dead Zone


(The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, a group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5-year average measured size target of 1,900 square miles.)

2019 Forecast: Summer Hypoxic Zone Size, Northern Gulf of Mexico

NOAA and the United States Geological Survey (USGS) released their 2019 forecast for the summer hypoxic zone size in the Northern Gulf of Mexico on June 10, 2019. Scientists are expecting the 2019 area of low oxygen, commonly known as the ‘Dead Zone,’ to be approximately 7,829 square miles, or about the size of Massachusetts. This prediction is large primarily because of high spring rainfall and river discharge into Gulf.