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Assessment of Vulnerability and Impacts to Global Change: Trophic State of Estuarine Systems

Assessment of Vulnerability and Impacts to Global Change: Trophic State of Estuarine Systems. Gustavo J Nagy. Problem The overload of nutrients: An environmental change on a global scale. The drivers of over-enrichment in N and P are the increase of population

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Assessment of Vulnerability and Impacts to Global Change: Trophic State of Estuarine Systems

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  1. Assessment of Vulnerability and Impacts to Global Change: Trophic State of Estuarine Systems Gustavo J Nagy

  2. Problem The overload of nutrients: An environmental change on a global scale The drivers of over-enrichment in N and P are the increase of population and economic activities (e.g. the use of synthetic fertilizers). Many estuarine ecosystems receive more nutrients than they can assimilate. The assessment of the vulnerability (susceptibility + sensitivity) of the trophic state of estuarine systems is under development.

  3. Eutrophication and Organic Metabolism Eutrophication is the process by which a body of water is enriched with organic material when this causes changes in that system. The excess of N and P stimulates the production of organic material in situ, which drives the autotrophy - heterotrophy balance (trophic state) to harmful levels. Eutrophication does not only depend on the nutrient load but also on physical factors, which are sensitive to climatic factors. Research Questions • Which are the drivers of eutrophication ? • How sensitive is the system to Climate Variability and Change ? • Which is the coping capacity of the ecosystem ?

  4. 1- The susceptibility (vulnerability) of estuarine systems to eutrophication depends on climatic conditions which govern the balance between buoyancy (river flow, temperature) and mixing forces (wind, tides).2- The export scenario for South America for the year 2050 would increase nearly three times. Generically, the temperate regions export less than half N from total anthropogenic sources than tropical regions do.This phenomenon is in part explained by lower precipitationENSOrelated variability plays a major role in controlling hydroclimatic drivers and associated state variables Why Eutrophication is related to Climate Change and Variability?

  5. Occurrence of eutrophication and physical factors • Occurrence of eutrophication effects indicates when the system cannot • cope with the available internal and or external nutrient inputs. • The most important factors in the expression of eutrophication are: • - the flushing time (f), • - the turbidity gradient, • - the nutrient inputs (NI) and consequent gradient of N, P and Si, • the mixing state, which in combination with fand NI may indicates • the sensitivity of any system to eutrophication.

  6. WIND TIDE (Q) RIVER FLOW Mixing Buoyancy ESTUARY > River Flow > Buoyancy (dS/dZ) “Stability” > Wind, Tide < Buoyancy “Mixing”

  7. Microtidal Systems 1) Susceptibility Mixing and transport processes in estuarine systems are dominated by tide (A), river flow (QF) and winds (W). Microtidal systems have a low mixing capacity, which depends mainly on the wind. River Flow is the “master variable” which controls buoyancy (dS/dZ), nutrification, time of residence, stratification and gravitational circulation, salinity and bottom oxygen depletion. The balance between these forces determines factors of organic metabolism and the degree of expression of eutrophication.

  8. 2) Determinant factors of eutrophication The comparative study of microtidal systems indicates that there are three main factors which affect susceptibility, all of which are very dependent on flow. 1- dilution (Q/VF), flushing time (f) and residence time (RT) of the estuarine water (Q/VE). 2- the ratio of the nutrient load QN by area (S) and volume (V). 3- the stratification-destratification cycle (mixing state).

  9. Development of a Highly Aggregated Trophic Status Index for the Rio de la Plata River Estuary Gustavo J Nagy, Graciela Ferrari, Juan J Lagomarsino, César H López and Alvaro Ponce  Problem We are in need to develop indicators of the trophic state (including vulnerability) of estuarine systems for both cross-system and long-term comparisons. Questions Which are the Best Indicators of Trophic State ? AAre they enough to assess TS under changing climate ? HHow do we weight values ? Goal  To develop a highly aggregate index of trophic status for the Rio de la Plata estuary

  10. Drivers and Pressures In the past century, the system was subject to an increase in human pressures: nutrification, change of use and erosion of the soil, and sewage outfalls, driven by the increase of human population and economic activities, and has also been subject to a great increase (25 to 35%) in precipitation and river flows (QF), driven by an increase in ENSO variability and the displacement towards the south of the subtropical Atlantic anticyclone. Atmospheric temperature increased 0.8º C, and it is estimated that the SST increased 1º C, due to the additional effect of the increase of QF (“freshwater is warmer”). These changes have been greater in the last two – three decades.

  11. Indicators of Trophic State The indicators of trophic state serve in the assessment of the impact of the degree of nutrient excess in estuaries. N and P are causal variables (of state), algal biomass (Chl-a), 02 and harmful algal blooms (HAB) are variables of response, or symptoms. d-PSIR FRAMEWORK DRIVERS PRESSURE STATE IMPACT RESPONSE Socioeconomic > Nutrient Load > N,P > HAB Trophic State Chl a Changes Climatic > Water load > Stratification < O2D Biodiversity

  12. The Rio de la Plata River Estuary System: Pressures and Trophic State The Rio de la Plata is a large (3.1 x 106 km2), microtidal (amplitude < 1.0 m) river estuary system, which is rich in nutrients and trophically based on plankton. It can be divided into tidal river and estuary (Figure 1) with different depth, stratification and circulation, water exchange, nutrient influx, turbidity and trophic state.

  13. Approach and Data In this presentation we focus on selected vulnerability and state /impact variables, emphasizing on Harmful Algal Blooms (HAB index). We used world-wide accepted criteria (NOAA, EPA, EEA, IFREMER) for N and P, Chl-a, 02 and HABs, adapted to available data. e.g. 3-4 classes or ranks of concentrations (better for cross-system comparison) Low Moderate High Very High All of the above mentioned reports prioritize average and /or extreme values attributable to typical climatic and environmental conditions on annual basis. However, in regions highly sensitive to ENSO-related hydroclimatic variability, the change of both flushing and nutrient inputs can modify the natural vulnerability and induce eutrophic conditions during extreme events.

  14. Variables and Index (Criteria and Framework) Our index will be based on two frameworks: 1) a d-PSIR key set of indicators (drivers, presure, state, impact and response). 2) an estuarine susceptibility key set of indicators (susceptibility  vulnerability). Variables 11)     susceptibility (e.g. residence time of water, degree of vertical stratification), 22)     drivers (population density and fertilizer use; climatic projections), 33)     pressure (N P Si load), 44)     state (e.g. N, P), 55)     impacts / symptoms(Chl-a), 02 and HABs occurrence).

  15. Results and Discussion - Symptoms of Trophic state The tidal river is characterized by:                 i.            an excess of N,                ii.            slight oxic deficit (<90% saturation)             iii.            cyanobacterial blooms. However, the expression of the symptoms of eutrophication is limited by the high turbidity of the water. The estuary is autotrophic and its nutrification is typical of non impacted systems   however, there are a number of eutrophic symptoms:               i.         oxygen deficit in the bottom layer (<60%) in conditions of prolonged stratification,               ii.         (sometimes), high biomass.              iii.         frequent HAB blooms in summer

  16. b- Extreme conditions and Coping Capacity During extreme El Niño (e.g. 1983) and La Niña (e.g. 1999) events, eutrophic levels of nitrate (> 50 M) and chl-a (> 20 g l-1) were measured respectively. The latter is also, an example of the coping capacity of estuarine systems. During normal flushing and residence time conditions, prevailing chlorophyll values are usually < 10 g l-1, whereas during very low river flow the increase in reside nce time of water and plankton induces a greater algal development. During very high QF both seaward displacement of the estuarine front and decreased residence time increase nutrient fluxes and export rate to the adjacent shelf.

  17. > N, P and > N/P • > dS/dZ • < Flushing Vulnerability Threshold ( > 32.000 m3/s) River Flow Coping range Vulnerability Threshold ( < 16.000 m3/s) • > Chl a • < dS/dZ • > Flushing • - > HABs

  18. Variables (symptoms of eutrophication) and Index (Operational) aa- Vulnerability variables  Example: Residence Time (Vulnerability & Coping Capacity)  Four classes are considered       low < 30 d      moderate 30-45 d (lower middle)      high 45-60 d (higher middle)      very high > 60 d bb- State variables Example: Dissolved nitrate (M) << 5 Null (0) 55-10 Very Low (1) 10-20 Low (2) 20-30Medium(3) 30-50High(4) >> 50 Very High (5) >> 100 Extreme (6)

  19. c-Impact variables State / impact: Oxygen saturation (%) Frequency average Frequent Sporadic Ø     100 saturation Null (0) Ø     90-100 saturation Very Low (1) Ø60-80Low (2) 2.0 Ø  50-60 Medium (3) Ø     40-50 High (4) 4x 0.15= 0.6 Ø     20-40 Very High / Severe (5) 5 x 0.10= 0.5 Ø     < 5 Extreme (6) Subtotal = 3.1

  20. HAB Index d- Impact Index: HAB Index HAB Index = Toxic Species Index + Noxious Species Index prevailingextreme Indicators 0 1 2 3 4 1 Intensity (cells/liter) 102 103 104 105 2 Persistance (months) 0.25 0.75 1-6 >6 3 Extension (% of coverage) 10 25 >50 >75 4 Toxicity (concentration)low mediumhigh very high

  21. Ssp indicators-IPETvalues from 0 to 4. Each species has the same weight. 0= absence 1= low 2= medium 3= high 4= very high Non weighted impact matrix for toxic species Weighting 1- Impact Health (0.75) > Economic (0.50) > Environment (0.25) 2- IPET Assessment for each species

  22. Non weighted Weighted

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