Cultural accelerated by anthropogenic activities

Transcription

1 EUTROPHICATION IMPLICATIONS OF N & P Intent of this lecture? Link our discussions of terrestrial N & P dynamics with its influences on receiving water bodies How the relative amounts of N & P can influence the growth and type of algae Making the connection between chemical pollution and biological problems Lecture based on Val H. Smith Cultural eutrophication of inland, estuarine, and coastal waters. In Successes, Limitations and Frontiers in Ecosystem Science. M. L. Pace & P. M. Groffman (Editors), Springer Verlag, NY Pgs And Aquatic Pollution Edward Laws This article highlights the important milestones in eutrophication research and provides an up to date discussion of N & P controls Natural versus Cultural Eutrophication Cultural accelerated by anthropogenic activities 1

2 Introduction brief history Productivity of water bodies research initiated in Europe work of Einar Naumann (Sweden) and August Theinemann (Germany) Naumann developed the Trophic State Concept o Phytoplankton production determined by concentrations of nitrogen and phosphorus o Productivity of lakes varied with the geological characteristics of the watersheds Naumann also developed the terms that we use to categorize lakes based on their productivity and nutrient supplies 1. OLIGOTROPHIC 2. MESOTROPHIC 3. EUTROPHIC 4. HYPEREUTROPHIC 2

3 Mean values for the trophic classification system Condition Total P (µg L -1 ) Chlorophyll a (µg L -1 ) Secchi disk depth (m) Ultra-Oligotrophic <4 <1 >12 Oligotrophic <10 <2.5 >6 Mesotrophic Eutrophic Hypereutrophic >100 >25 <1.5 3

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7 Eutrophication studies began in the 1940s and published work peaked in the 1980s / 90s. These studies were conducted via Comparative analyses of different water bodies over time and space, and Experimentation at three different scales 1. small-scale flask experiments such as bio-assays 2. medium-scale experiments such as mesocosms 3. whole-system manipulations (whole-lake experiments) 7

8 Nitrogen & Phosphorus as limiting nutrients First important development in eutrophication research identification of N & P Driven by Leibig s Law of the Minimum (1855) The yield of a given species should be limited by the nutrient that was present in the least quantity in the environment relative to its demands for growth Variables for eutrophication Sunlight Silica N P Leibig s law resulted in the concept of nutrient limitation for algal growth 1. a single nutrient should be the primary limiting factor for algal growth 2. observed algal growth should be proportional to the supply of the limiting nutrient 3. Practical control of the growth of algae can be accomplished via restricting the supply of the nutrient. 8

9 N & P were identified as the primary limiting nutrients -- this recognition was arrived at primarily by using bioassays. Other approaches used Physiological indicators Elemental ratios the ratio of N and P concentrations in the vegetation Redfield (1958) proposed that the nutrient content of living algal cells can be given by a 7:1 by mass for N:P, or elemental ratio of 16:1 for N:P -- referred to as the Redfield ratio Given mass ratio, how would you compute the elemental ratio? This concept was extended to determine N & P controls If N:P supply > Redfield ratio P limiting If N:P supply < Redfield ratio N limiting However later work showed that algae are composed of many species with varying N:P ratio (3 30) and thus a single ratio was not very effective in determining N or P limitation 9

10 This variability was evident in Sakamoto s (1966) results when TN:TP < 10:1 N limitation when TN:TP > 17:1 P limiting between 10:1 17:1 either N or P What implications does this have? 10

11 Nutrient Loading Models (2 nd important development) Development of mass budgets for N & P for water bodies and the linkage of input/outputs with waterbody concentrations Initially received with considerable skepticism; Thienemann remarked that the concept had nothing to do with limnology. Vollenweider, Reckhow and Chapra leading proponents of the mass balance approach Mass balance approaches since then have become a cornerstone of ecosystem science research Models are based on the hydraulic characteristics of the water body depth, flow rate, length, etc. factors that 11

12 determine the residence time or turnover time of the waterbody P models have received most of the attention. Example of P model TP in lake P concentration (mg/m 3 ) P TP = in t w 0.82 where P in is the mean annual P concentration in inflow, and t w is the residence time (yrs) how would you compute the residence time of a lake? Compared to N, P models have also been easier to formulate why? Also we have had more success in formulating mass balance models for freshwater systems than marine systems -- difficult to identify boundaries of marine water bodies; considerable mixing of waters 12

13 Effects of nutrients on aquatic systems 3 rd important development Linking nutrient concentrations to water quality variables of concern e.g., algal biomass, water clarity, etc. Our perception/classification of eutrophication for freshwater and marine waters varies! NOTE lower thresholds (chlorophyll, clarity) for marine waters 13

14 The controls that N and P have for eutrophication also varies with these waters Most of the freshwater lakes and reservoirs are limited by P Strong relationships have been found between the concentrations of P and algal mass (measured sometimes as the chlorophyll-a content) Figure data for phytoplankton floating algae in water bodies 2 observations from the figure hyperbolic relationship with P, with decreasing algal yields beyond TP > 100 mg/m 3 14

15 Figure also indicates the shifts in limiting nutrient (becomes N limited) as P concentration increases Similar controls of P for periphyton (algae attached to substrate) in streams and rivers have also been observed periphyton in Scajaquada Creek in Buffalo (2002). Figure

16 Suspended algae in streams also show P influences. These algae originate from periphyton. suspended algae in Scajaquada Creek in Buffalo (2002) 16

17 However the mass of suspended algae in streams at a given level of P is much lower than the equivalent biomass of phytoplankton in lakes 17

18 In contrast, in marine systems the phytoplankton biomass is more tied to the N concentrations suggesting that these systems are N-limited Why? Likely reasons 1. P sequestration by Fe and Al oxides in freshwater systems 2. Fe and Mo availability for N-fixing bacteria in freshwater systems (Fe and Mo are very low in marine waters) 3. High losses of N due to denitrification in marine waters However, some studies (Hecky & Kilham, 1988) have shown that N-limitation may not be applicable always. Inability to define a well contained system has also limited a more thorough understanding of the N and P controls on marine phytoplankton 18

19 N & P Mixing issues of stratification and overturning Stratification of the lake because of the water temperature gradients during seasons Stratification occurs because of the variation in the density of water with temperature - water most dense at 4 degree C - less dense above and below 4 degree C 19

20 Mixing is limited to specific zones Surface mixing occurs in the epilimnion maybe 1m deep in summer or as much as 100 m under destabilized conditions. Epilimnion water will not mix with the hypolimnion when stratification is in place. After summer - When surface water temperatures cool up reach 4 degree C the surface water sinks causes mixing and overturning! --- Fall Overturn If surface temps drop below 4C, again stratification will take place 20

21 In spring when surface waters warm up and go beyond 4C you will have the spring overturn! Two overturning periods dimictic One overturning period monomictic Two important functions of overturning: 1. oxygenation of the hypolimnion by sinking surface waters 2. nutrient enrichment of the epilimnion by rising bottom waters Nutrient availability with autumn and spring overturns Table: 21

22 Susceptibility of water bodies to O 2 depletion: Deep oligotrophic systems do NOT suffer depletion Low productivity in epilimnion respiratory rates not high enough The large depth of hypolimnion provides more than enough O 2 to meet any likely demands Shallow eutrophic systems do not develop seasonal depletion because the mixing layer extends to the bottom But shallow eutrophic systems may develop low O 2 conditions overnight under no wind or calm weather conditions (when the mixing mechanism is shut off) --- example the shallow western basin of Lake Erie Eutrophic systems with intermediate depths most likely to be seasonally depleted by O 2 - hypolimnion is small in depth - high production in the epilimnion - e.g., - central basin of Lake Erie. Seasonal depletions of O 2 do not cause mass die off s of fish and aquatic species it s the overnight O 2 depressions that do! 22

23 Salinity gradients can also affect mixing/stratification Occurs in estuaries e.g, the Chesapeake Bay. May occur in conjunction with the temperature gradients and make the scenario even worse. In the Chesapeake Bay increased inflows of freshwater make the O2 depletion condition worse. Increasing urbanization (impervious surfaces) are expected to increase flows. 23

24 Chesapeake Bay N and P limitations Vary with season! Late winter/early spring greatest outflows in the bay. Runoff rich in N from fertilizer applications Most severe phytoplankton blooms occur in late winter/spring N:P ratio of water can be as high - 90:1 P limiting conditions! Summer Runoff from watersheds is lowest Internal P release from bottom sediments due to bacterial decomposition N:P ratio of bay water can be as low as 5:1 N limits phytoplankton growth! Hypoxia in the bay number of factors Nutrients 24

25 Freshwater runoff in spring Oysters shell fish filter feeders of bacteria (bacteria responsible too for oxygen depletion!) 25

26 N & P influences on phytoplankton community structure The relative concentrations of N & P not only effect the biomass but can also cause shifts in the species type Cynobacteria blooms have been associated with N limiting conditions (Cynobacteria are bacteria with chlorophyll and NOT a class of algae) cynobacteria mass o Problem with cynobacteria is that these species produce potent toxins that impact other aquatic species o Cynobacteria have also been found under excessive eutrophic conditions figure

27 The resource-ratio theory has been used to explain the appearance of cynobacteria under N limiting conditions o Resource-ratio theory supplies of N, P, and silicon and light are important determinants of algal species composition Under N limiting conditions cynobacteria that are better N scavengers than other species, out-compete the other algal species. Blue-green algae can fix their own N! 27

28 This relationship is highlighted in Figure 2.7 (relative cynobacterial biomass increases for lower N:P ratios) Implications of low N:P for plankton type???? 28

29 Management Implications for N:P ratio loadings Virginia Coastal Plain Example BMP implementation decreased the N:P ratio of exports from the watershed much more needs to be done for P control total-p total-n particulate-p soluble-org-p ortho-p pre-bmp post-bmp particulate-n soluble-org-n nitrate-n ammon-n loads (kg/ha) 29

30 N:P ratio QN1 pre-bmp post-bmp NOTE: elemental N : P ratio 30