Underwater light climate and wavelength dependence of microalgae photosynthetic parameters in a temperate sea

Studying how natural phytoplankton adjust their photosynthetic properties to the quantity and quality of underwater light (i.e. light climate) is essential to understand primary production. A wavelength-dependent photoacclimation strategy was assessed using a multi-color pulse-amplitude-modulation chlorophyll fluorometer for phytoplankton samples collected in the spring at 19 locations across the English Channel. The functional absorption cross section of photosystem II, photosynthetic electron transport (PETλ) parameters and non-photochemical quenching were analyzed using an original approach with a sequence of three statistical analyses. Linear mixed-effects models using wavelength as a longitudinal variable were first applied to distinguish the fixed effect of the population from the random effect of individuals. Population and individual trends of wavelength-dependent PETλ parameters were consistent with photosynthesis and photoacclimation theories. The natural phytoplankton communities studied were in a photoprotective state for blue wavelengths (440 and 480 nm), but not for other wavelengths (green (540 nm), amber (590 nm) and light red (625 nm)). Population-detrended PETλ values were then used in multivariate analyses (partial triadic analysis and redundancy analysis) to study ecological implications of PETλ dynamics among water masses. Two wavelength ratios based on the microalgae saturation parameter Ek (in relative and absolute units), related to the hydrodynamic regime and underwater light climate, clearly confirmed the physiological state of microalgae. They also illustrate more accurately that natural phytoplankton communities can implement photoacclimation processes that are influenced by in situ light quality during the daylight cycle in temporarily and weakly stratified water. Ecological implications and consequences of PETλ are discussed in the context of turbulent coastal ecosystems.

Photoprotective compounds in zooplankton of two adjacent tropical high mountain lakes with contrasting underwater light climate and fish occurrence

We compared temporal changes in the concentration of photoprotective compounds (PCs) in zooplankton of two adjacent (600 m apart) tropical high mountain lakes, El Sol and La Luna. The lakes have the same geological origin, age and drainage basin. However, they differ in underwater light and UV penetration and in the presence of Oncorhynchus mykiss, only found in El Sol. Whereas Daphnia ambigua from the clear La Luna had carotenoids and melanin, in El Sol, the same species lacked pigmentation. The copepod Leptodiaptomus cuauhtemoci was only found in El Sol and relied on accumulation of MAAs and carotenoids. The maximum concentration of PCs was found over the period with highest incident ultraviolet radiation (UVR). Our data suggest that tropical species of zooplankton respond to UVR by accumulating PCs that are too different in relation to exposure to UVR. However, we cannot discard the possibility that the presence of fish in Lake El Sol influences also their phenotypical and behavioral responses. Overall, zooplankton exhibited an ample phenotypic plasticity in the type and concentrations of PCs in response to the high and variable doses of UVR prevailing in low-latitude lakes, similar to the zooplankton of their high-latitude counterparts.

John Francis Talling. 23 March 1929—20 June 2017

John (Jack) Francis Talling was a master limnologist who pioneered much of our understanding of the River Nile, the great lakes of the African Rift Valley and those of the English Lake District. He was one of the world's leading authorities on the ecophysiology of freshwater phytoplankton and specialized in the control of their productivity by light and carbon dioxide. His perspectives were formed by interaction with leading scientists of the day, mainly at the Freshwater Biological Association, Cumbria, but also at laboratories in Africa and at the Scripps Institution of Oceanography, California. Jack's work on the effects of light on phytoplankton productivity was based on detailed measurements of the underwater light climate in lakes and oceans as well as laboratory and field measurements of the response of phytoplankton photosynthesis to light, involving the development of new or improved measurement methods. Calculation procedures were developed to estimate how light controls primary productivity. He devised the widely used characteristic ‘ I k ’ to quantify the onset of light saturation in the curve that defines the response of photosynthesis to light quantity. Experiences in extremely productive African soda lakes stimulated an interest in the possibility of CO 2 being a limiting factor controlling phytoplankton productivity and, more generally, Jack had the ecological insight to recognize that ecological dynamics resulted from interactions among factors rather than a response to a single variable.

Different responses of primary producers to mild winter in macrophyte- and phytoplankton-dominated lakes

Early ice-out on lakes or the absence of ice cover can lead to intense water mixing, high turbidity, and changes in nutrient balance and light conditions in the water, which in turn affect the phytoplankton community and submerged aquatic vegetation (SAV) in the aquatic environment. This study aimed to evaluate whether the effects of a mild winter on these primary producers are differentiated in two types of eutrophic lakes: phytoplankton-dominated (PDL), with a low proportion of SAV, and macrophyte-dominated (MDL), with extensive benthic vegetation. The physical and chemical parameters of the water, concentration of chlorophyll a, and the biomass of phytoplankton and SAV were investigated in four small eutrophic lakes (Eastern Poland) during the growing season after cold winters (CW) and mild winters (MW). In both types of lakes, the concentrations of soluble and total nitrogen were higher and the values of soluble and total phosphorus were lower after the MW than after the CW. The concentration of chlorophyll a and the phytoplankton biomass were always higher in the PDL than in the MDL, but the increase in both parameters in the vegetation season after the MW was statistically important only in the PDL. A poor underwater light climate was stressful for the SAV in the PDL, but not in the MDL. The MW had negative effects on phytoplankton diversity and the loss of light-demanding macrophytes. However, the biomass of primary producers in the MDL seems to be more resistant to climate warming, in comparison to the PDL.

Water quality and potamoplankton of the river Buriganga and Gomti: A comparison

The studied section of the river Buriganga besets with urban catchment contained higher values of pH, alkalinity, CO2, conductivity, TDS, SRP, SRS and NO3-N concentration compared to its counterpart, namely a section of the river Gomti having rural catchment characteristics. The mean values for a period of six months for the river Buriganga were 8.34, 1.48 meq/l, 8.49 mg/l, 686 μS/cm, 155.17 mg/l, 493 μg/l, 36.07 mg/l and 810.28 μg/l for pH, alkalinity, CO2, conductivity, TDS, SRP, SRS and NO3-N, respectively. While the same for the river Gomti was 7.86, 1.18 meq/l, 5.42 mg/l, 284.44 μS/cm, 79.91 mg/l, 188.37 μg/l, 26.41 mg/l and 203.99 μg/l, respectively for pH, alkalinity, CO2, conductivity, TDS, SRP, SRS and NO3-N. In the river Gomti, the concentration of DO was better (7.87 mg/l) compared to that of river Buriganga (5.53 mg/l). Because of a hilly origin of the river Gomti and availability of sediments in its water, the underwater light climate was poor compared to the river Buriganga. The Secchi depth recorded for Gomti and Buriganga were 0.26 and 0.54 m, respectively. Both the rivers were found diatom dominant in terms of population but the species prevailed were different. Fragillaria virescens Ralfs was the dominant diatom of the river Buriganga. Its density ranged from 6.5-11.09×105 ind/l which was the highest compared to the population of all other groups of potamoplankton. In Gomti another diatom namely, F. pinnata Ehrenberg was dominant and the density of which ranged from 8.45 - 12.65 × 105 ind/l. Community structure of the potamoplankton revealed 30 species from the river Buriganga and 22 species from Gomti. The study reveals that relatively higher concentration of nutrients prevails in the water of the river course having urban catchment characters compared to the rural ones. Dhaka Univ. J. Biol. Sci. 27(2): 191-200, 2018 (July)

Effects of autotrophic biomass and composition on photosynthesis, respiration and light utilisation efficiency for a tropical savanna river

The efficiency of light used for photosynthesis, when standardised for areal chlorophyll (Chl)-a biomass, is summarised by the light utilisation efficiency parameter and is dependent on light at the water’s surface, the underwater light climate and autotroph characteristics. Herein we examined the relationships between light, photosynthesis, respiration and autotroph biomass in a tropical savanna river in northern Australia during the dry season when autotroph biomass accumulated following wet season disturbance. The river’s autotrophs comprised mainly benthic microalgae, macroalgae and macrophytes. Total Chl-a and dry weight biomasses increased 4- and 27-fold respectively over 5 months, whereas photosynthesis doubled. Photosynthesis was light limited and, when standardised for Chl-a and dry weight biomasses, declined with increasing biomass, despite increasing incident light through the study period. We surmised this was due to self-shading and autotrophic composition, which had variable Chl-a content and resulted in a 10-fold reduction in the light utilisation efficiency with increasing light and biomass. Because respiration was tightly coupled to photosynthesis, biomass-standardised respiration also decreased with increasing biomass. Autotrophic self-shading and composition can have significant effects on light utilisation efficiency and the biomass–photosynthesis relationship, and warrant consideration when interpreting photosynthesis for river health monitoring.