Modeling the irradiance dependency of the quantum efficiency of potosynthesis

Measures of the quantum efficiency of photosynthesis (phi(PSII)) across an irradiance (E) gradient are an increasingly common physiological assay and alternative to traditional photosynthetic-irradiance (PE) assays. Routinely, the analysis and interpretation of these data are analogous to PE measurements. Relative electron transport rates (rETR = E x phi(PSII)) are computed and fit to a PE curve to retrieve physiologically meaningful PE parameters. This widespread approach is statistically flawed as the response variable (rETR) is explicitly dependent on the predictor variable (E). Alternatively the E-dependency of phi(PSII) can be modeled directly while retaining the desired PE parameters by normalizing a given PE model to E. This manuscript presents a robust analysis in support of this alternative procedure. First, we demonstrate that scaling phi(PSII) to rETR unnecessarily amplifies the measurement error of phi(PSII) and using a Monte-Carlo analysis on synthetic data induces significantly higher uncertainty in computed PE parameters relative to modeling the E-dependency of phi(PSII) directly. Next a large dataset is simultaneously fitted to four PE models implemented in their original and E-normalized forms. Four statistical criteria used to evaluate the efficacy of nonlinear models demonstrate improved model fits and more precise PE parameters when data are modeled as E-dependent changes in phi(PSII). The analysis presented in this manuscript clearly demonstrates that modeling the E-dependency of phi(PSII) directly should be the norm for interpreting active fluorescence measures

Phytoplankton Production In Lake Victoria, East Africa

This thesis develops, validates and applies an empirical model that provides the first spatially explicit estimates of gross and net phytoplankton production in Lake Victoria. Gross and net phytoplankton production are in turn used to estimate the maximum sustainable yield (MSY) of Lake Victoria's fishery following an empirical formula and the carbon efficiency transfer method. Chapter 2 presents results from three inshore areas where diurnal and sub-seasonal gross and net phytoplankton production was derived using an adapted version of the phytoplankton production model developed by Fee (1990). Spatial and temporal trends of chlorophyll (chl), PI parameters, the vertical attenuation of PAR (kPAR), Secchi depths (SD) and respiration rates are identified. kPAR and SD are highly correlated to chl within the euphotic zone, as well as to each other. Furthermore, the two PI parameters, PBM and aB, exhibit a strong linear relationship and both decline along an increasing chl gradient, presumably due to increased light-limitation, a taxonomic shift from diatoms to cyanobacteria with increasing chl as well as an increased need for biologically fixed nitrogen. These hypotheses are supported by observed synchronous changes in the PSII:PSI ratio of phytoplankton and changes in the chl-specific attenuation of PAR (kchl). Relationships are also derived between biomass-specific respiration rates (RB) with chl and PBM; similar to PI parameters RB decreases with increasing chl. Owing to these correlative trends, only one parameter is required to estimate gross phytoplankton production through the empirical model developed in this thesis. The empirical model predicts that gross phytoplankton production increases in a near linear fashion between chl of 0 to 10 mg. m-3, begins to flatten out as chl approaches 20 mg. m-3 and then slightly decreases when chl exceeds 40 mg. m-3 where the maximum PPG of 13. 1 g O₂. m-2. day-1 is reached and is in close agreement with a theoretical argument proposed by Talling (1965). Areal respiration and consequently net phytoplankton production are sensitive to chl within the mixed layer as well as mixed layer depths. Overall, the lakewide averages of gross and net phytoplankton production are 9. 68 and 2. 2 g O₂. m-2. day-1 respectively. Significant temporal variability was observed on sub-seasonal scales within the inshore of Lake Victoria, and changes in limnological parameters coincided with changes in water column temperatures in each of the three bays. In Fielding Bay, the availability of meteorological data revealed that strong nocturnal wind events decreased both the water column temperature and chl, while both parameters generally increased in the absence of any such wind event. Lateral exchange of water with deeper areas through strong wind events essentially flushes Fielding Bay causing the observed decreases in both the water column temperature and chl; this hydrodynamic event also influences other limnological parameters according to their respective correlative regression equations with chl. Spatial trends were also observed between inshore areas. The deepest area, Napoleon Gulf, has the lowest values of chl while the shallowest area, Inner Murchison Bay, has the highest chl as the mean depth of a bay sets an approximate upper limit on chl. With respect to diurnal variability, PI parameters decline through the day, kPAR increases over the day and no statistically valid trends were ascertained for chl and RB. Chapter three examined spatial and seasonal patterns of chlorophyll fluorescence, temperature, dissolved oxygen and water transparency from four lakewide cruises. Significant spatial variability of each parameter confirmed that lakewide data is required to generate spatially explicit estimates of phytoplankton production. Complex patterns in the thermal structure during each cruise illustrated that physical processes in Lake Victoria are at times more complex that a previously stated unidirectional hypothesis of warm water in the north and cool water in the south (Spigel and Coulter 1996), and these patterns influence spatial patterns in dissolved oxygen and Secchi depths. Similar to Chapter 2, estimates of chl within the mixed layer were highly correlated to mixed depths, while lakewide averages of chl are lower than previously reported offshore values (Mugidde 1993, 2001)

Bio-optical Modeling of Aquatic Photosynthesis in the Laurentian Great Lakes

The methodology of phytoplankton production measurements in the Laurentian Great Lakes and other freshwater lakes has remained largely unchanged in the past 40 years. In most studies photosynthesis from a single water sample is measured across an in vitro light gradient usually using an artificial light source then extrapolating to the in situ environment. These traditional methods are laborious, thus limiting the amount of observations in space and time, and may not accurately represent in situ photosynthesis. Active chl a fluorescence, intrinsically linked to photosynthesis, can be measured in situ and instantaneously. Various bio-optical models that scale these fluorescence measurements to phytoplankton production are gaining widespread attention in the marine environment but have not been extensively tested in freshwater ecosystems. The methodology and efficacy of the various bio-optical models are tested in this thesis using a large dataset of active fluorescence profiles and ancillary water chemistry parameters against synchronously derived in vitro phytoplankton production collected across mixing, trophic and taxonomic gradients in Lake Erie. From this analysis, the most common bio-optical model parameterization yields photosynthetic rates that are largely incongruent with in vitro measurements. Bio-optical models are largely a function of two parameters, the absorption spectrum of photosystem II (aPSII) and the photochemical efficiency of PSII (fPSII). In Lake Erie fPSII is relatively constrained suggesting that even nutrient limited phytoplankton achieve balanced growth by adjusting the supply of energy through changes in light harvesting (aPSII) to match the demand for photosynthetic energy. This thesis goes on to demonstrate the success of bio-optical models depends largely on the formulation of aPSII. Alternative methods to derive aPSII, largely ignored in published bio-optical models, are reviewed, formulated, and when incorporated into a bio-optical model and compared to synchronous in vitro production measurements, this novel bio-optical model outperforms all other comparative studies performed across a taxonomic gradient

The Effect of Fertilization on Biomass and Metabolism in North Carolina Salt Marshes: Modulated by Location-Specific Factors

The resilience of salt marshes to sea level rise depends on vertical accretion through belowground biomass production and sediment deposition to maintain elevation above sea level. Increased nitrogen (N) availability from anthropogenic sources may stimulate aboveground biomass production and sediment deposition and, thus, accretion; however, increased N may also negatively impact marsh accretion by decreasing belowground biomass and increasing net CO2 emissions. A study was conducted in Spartina alterniflora‐dominated salt marshes in North Carolina, USA, to determine how responses to fertilization vary across locations with different physical and chemical characteristics. Pore water residence time, inundation time, and proximity to tidal creeks drove spatial differences in pore water sulfide, ammonium, and dissolved carbon concentrations. Although annual respiration and gross primary production were greater at the creek edge than interior marsh sites, net ecosystem CO2 exchange (NEE) was nearly balanced at all the sites. Fertilization decreased belowground biomass in the interior sites but not on the creek edge. Aboveground biomass, respiration, gross primary production, and net CO2 emissions increased in response to fertilization, but responses were diminished in interior marsh locations with high pore water sulfide. Hourly NEE measured by chambers were similar to hourly NEE observed by a nearby eddy covariance tower, but correcting for inundation depth relative to plant height was critical for accurate extrapolation to annual fluxes. The impact of fertilization on biomass and NEE, and thus marsh resilience, varied across marsh locations depending upon location‐specific pore water sulfide concentrations

Toward autonomous measurements of photosynthetic electron transport rates: An evaluation of active fluorescence-based measurements of photochemistry

This study presents a methods evaluation and intercalibration of active fluorescence-based measurements of the quantum yield (ϕʹ 0 PSII) and absorption coefficient (aPSII) of photosystem II (PSII) photochemistry. Measurements of ϕʹ PSII , aPSII, and irradiance (E) can be scaled to derive photosynthetic electron transport rates (P e ), the process that fuels phytoplankton carbon fixation and growth. Bio-optical estimates of ϕ PSII and aPSII were evaluated using 10 phytoplankton cultures across different pigment groups with varying bio-optical absorption characteristics on six different fast-repetition rate fluorometers that span two different manufacturers and four different models. Culture measurements of ϕʹ PSII and the effective absorption cross section of PSII photochemistry (σPSII, a constituent of aPSII) showed a high degree of correspondence across instruments, although some instrument-specific biases are identified. A range of approaches have been used in the literature to estimate aPSII(λ) and are evaluated here.With the exception of ex situ aPSII(λ) estimates from paired σPSII and PSII reaction center concentration ([RCII]) measurements, the accuracy and precision of in situ aPSII(λ) methodologies are largely determined by the variance of method-specific coefficients. The accuracy and precision of these coefficients are evaluated, compared to literature data, and discussed within a framework of autonomous P e measurements. This study supports the application of an instrument-specific calibration coefficient (KR) that scales minimum fluorescence in the dark (F 0 ) to aPSII as both the most accurate in situ measurement of aPSII, and the methodology best suited for highly resolved autonomous P e measurements

Improving the Accuracy of Single Turnover Active Fluorometry (STAF) for the Estimation of Phytoplankton Primary Productivity (PhytoPP)

Photosystem II (PSII) photochemistry is the ultimate source of reducing power for phytoplankton primary productivity (PhytoPP). Single turnover active chlorophyll fluorometry (STAF) provides a non-intrusive method that has the potential to measure PhytoPP on much wider spatiotemporal scales than is possible with more direct methods such as 14C fixation or O2 evolved through water oxidation. Application of a STAF-derived absorption coefficient for PSII light-harvesting (aLHII) provides a method for estimating PSII photochemical flux on a unit volume basis (JVPII). Within this study, we assess potential errors in the calculation of JVPII arising from sources other than photochemically active PSII complexes (baseline fluorescence) and the package effect. Although our data show that such errors can be significant, we identify fluorescence-based correction procedures that can be used to minimize their impact. For baseline fluorescence, the correction incorporates an assumed consensus PSII photochemical efficiency for dark-adapted material. The error generated by the package effect can be minimized through the ratio of variable fluorescence measured within narrow wavebands centered at 730 nm, where the re-absorption of PSII fluorescence emission is minimal, and at 680 nm, where re-absorption of PSII fluorescence emission is maximal. We conclude that, with incorporation of these corrective steps, STAF can provide a reliable estimate of JVPII and, if used in conjunction with simultaneous satellite measurements of ocean color, could take us significantly closer to achieving the objective of obtaining reliable autonomous estimates of PhytoPP

The Nile perch invasion in Lake Victoria: cause or consequence of the haplochromine decline?

We review alternative hypotheses and associated mechanisms to explain Lake Victoria’s Nile perch takeover and concurrent reduction in haplochromines through a (re)analysis of long term climate, limnological and stock observations in comparison with size-spectrum model predictions of co-existence, extinction and demographic change. The empirical observations are in agreement with the outcomes of the model containing two interacting species with life-histories matching Nile perch and a generalized haplochromine. The dynamic interactions may have depended on size related differences in early juvenile mortality: mouth-brooding haplochromines escape predation mortality in early life stages, unlike Nile perch that have miniscule planktonic eggs and larvae. In our model predation on the latter by planktivorous haplochromine fry act as a stabilizing factor for co-existence, but external mortality on the haplochromines would disrupt this balance in favor of Nile perch. To explain the observed switch, mortality on haplochromines would need to be much higher than the fishing mortality that can be realistically re-constructed from observations. Abrupt concomitant changes in algal and zooplankton composition, decreased water column transparency, and widespread hypoxia from increased eutrophication most likely caused haplochromine biomass decline. We hypothesize that the shift to Nile perch was a consequence of an externally caused, climate triggered, decrease in haplochromine biomass and associated recruitment failure rather than a direct cause of the introduction

Multiple stressors cause rapid ecosystem change in Lake Victoria

1. Lake Victoria endured multiple stresses over the past century including population growth, increased cultivation of land, meteorological variability, resource extraction, intensive fishing, introduction of exotic species and more recently climate warming. These stressors became manifest through a fundamental and rapid change in the fish community and fishery in the early 1980s and visible eutrophication. However, the relation of these two phenomena and the possible interaction of the multiple stressors have been difficult to establish because of the temporally fragmented nature of the environmental data.2. Comprehensive limnological observations from the 1960s were repeated in the 1990s and established the eutrophication of the lake, but these do not provide insight to the time course of when changes in trophic state occurred. Comprehensive fishery catch data from 1965 to the present provide a time course of the change in community composition and yield but cannot be correlated in time with discontinuous and sparse limnological data to determine possible cause–effect relationships.3. Palaeolimnologic studies were conducted on three cores, two offshore and one nearshore, to establish a time course for the eutrophication of the lake that can be related to time-based data on the fishery. In the 1920s, the cores recorded an increase in nitrogen content of the sediments, but there was no significant response in the paleo-productivity indicators of biogenic Si deposition and change δ 13 C of deposited organic matter. Phosphorus deposition began to increase in the 1940s in all three cores after which biogenic Si deposition increased steadily over time. Responses in δ 13 C of organic matter begin in the 1960s at the coring sites. In the 1970s, the δ 13 C of organic matter at the nearshore site increased nearly 3‰ in a 10-year period likely as a response to a dramatic increase in internal P loading caused by spreading anoxia.4. Nile perch, the large predatory fish introduced in 1954, had become established through much of the lake at low abundances by the 1970s. In 1980, the catch of this fish began to increase, and by the end of the decade, the Lake Victoria fishery was the largest lake fishery in the world; and Nile perch dominated the catch. While catches of some other fishes also increased, the endemic haplochromines suffered a catastrophic decline in abundance and loss of biodiversity.5. The detailed chronostratigraphies for these sediment cores established that the major changes in the trophic condition of the lake were accomplished prior to the change in the fish community and that the increased primary productivity of the lake likely contributed to the increased fish catches after 1980. The increased algal abundance also would have greatly reduced visibility and facilitated the emergence of Nile perch as the dominant top predator.6. Thematic implications : multiple stresses were present in Lake Victoria over several decades, but transition to a new ecosystem state with a transformed food web and highly productive algal community may have been triggered by a period of low wind stress and then generally warming climate since the 1970s. Unless phosphorus loading is stabilised or reduced, the ecosystem’s diversity and balanced productivity will not recover, and other state transitions may occur to the detriment of the lake and its riparian populations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78648/1/j.1365-2427.2009.02374.x.pd

Як уникнути підйому рівня води?

East Africa’s Lake Victoria provides resources and services to millions of people on the lake’s shores and abroad. In particular, the lake’s fisheries are an important source of protein, employment, and international economic connections for the whole region. Nonetheless, stock dynamics are poorly understood and currently unpredictable. Furthermore, fishery dynamics are intricately connected to other supporting services of the lake as well as to lakeshore societies and economies. Much research has been carried out piecemeal on different aspects of Lake Victoria’s system; e.g., societies, biodiversity, fisheries, and eutrophication. However, to disentangle drivers and dynamics of change in this complex system, we need to put these pieces together and analyze the system as a whole. We did so by first building a qualitative model of the lake’s social-ecological system. We then investigated the model system through a qualitative loop analysis, and finally examined effects of changes on the system state and structure. The model and its contextual analysis allowed us to investigate system-wide chain reactions resulting from disturbances. Importantly, we built a tool that can be used to analyze the cascading effects of management options and establish the requirements for their success. We found that high connectedness of the system at the exploitation level, through fisheries having multiple target stocks, can increase the stocks’ vulnerability to exploitation but reduce society’s vulnerability to variability in individual stocks. We describe how there are multiple pathways to any change in the system, which makes it difficult to identify the root cause of changes but also broadens the management toolkit. Also, we illustrate how nutrient enrichment is not a self-regulating process, and that explicit management is necessary to halt or reverse eutrophication. This model is simple and usable to assess system-wide effects of management policies, and can serve as a paving stone for future quantitative analyses of system dynamics at local scales

Basin-Scale Control on the Phytoplankton Biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria