Increases in Picocyanobacteria Abundance in Agriculturally Eutrophic Pampean Lakes Inferred from Historical Records of Secchi Depth and Chlorophyll-a

Phytoplankton size structure has profound consequences on food-web organization and energy transfer. Presently, picocyanobacteria (size < 2 µm) represent a major fraction of the autotrophic plankton of Pampean lakes. Glyphosate is known to stimulate the development of picocyanobacteria capable of degrading the herbicide. Due to the worldwide adoption of glyphosate-resistant crops, herbicide usage has increased sharply since the mid-1990s. Unfortunately, there are very few studies (none for the Pampa region) reporting picocyanobacteria abundance before 2000. The proliferation of µm sized particles should decrease Secchi disc depth (ZSD). Therefore ZSD, conditional to chlorophyll-a, may serve as an indicator of picocyanobacteria abundance. We use generalized additive models (GAMs) to analyze a “validation” dataset consisting of 82 records of ZSD, chlorophyll-a, and picocyanobacteria abundance from two Pampean lakes surveys (2009 and 2015). In support of the hypothesis, ZSD was negatively related to picocyanobacteria after accounting for the effect of chlorophyll-a. We then fitted a “historical” dataset using hierarchical GAMs to compare ZSD conditional to chlorophyll-a, before and after 2000. We estimated that ZSD levels during 2000–2021 were, on average, only about half as deep as those during 1980–1999. We conclude that the adoption of glyphosate-resistant crops has stimulated outbreaks of picocyanobacteria populations, resulting in lower water transparency.

Relationship between turbidity and the benthic community in the preserved Montebello Lakes in Chiapas, Mexico

One of the consequences of increased turbidity in lakes is the modification of the structure of the benthic macroinvertebrate community. To understand this relationship, we evaluated 13 lakes in the Montebello Lakes National Park in Chiapas. The lakes have been affected by gradual eutrophication over decades, producing variable transparency values among lakes. Macroinvertebrates were sampled from each lake in the rainy and dry seasons, and species richness and abundance were calculated and related to Secchi disc transparency. Cluster analysis showed that community composition was similar in pristine and semitransparent lakes, in contrast with turbid lakes. Considering macroinvertebrate groups, hyalellids were dominant in pristine and semiturbid lakes, whereas chironomids were dominant in turbid lakes. A significant quadratic relationship between richness and Secchi disc depth values was found, which is consistent with the intermediate production hypothesis. This study shows how a gradual change in Secchi disc depth can markedly modify benthic communities.

Turbidez y profundidad de disco de Secchi con Sentinel-2 en embalses con diferente estado trófico en la Comunidad Valenciana

<p>Transparency or turbidity is one of the main indicators in studies of water quality using remote sensing. Transparency can be measured <em>in situ</em> through the Secchi disc depth (SD), and turbidity using a turbidimeter. In recent decades, different relationships between bands from different remote sensing sensors have been used for the estimation of these variables. In this paper, several indices and spectral bands have been calibrated in order to estimate transparency from Sentinel-2 (S2) images from field data, obtained throughout 2017 and 2018 in Júcar basin reservoirs with a great variety of trophic states. Three atmospheric correction methods developed for waters have been applied to the S2 level L1C images taken at the same day as the field data: Polymer, C2RCC and C2X. From the spectra obtained from S2 and the SD field data, it has been found that the smallest error is obtained with the images atmospherically corrected with Polymer and a potential adjustment of the reflectivities’ ratio of the blue and green bands (R₄₉₀/R₅₆₀), which allow the estimation of SD with a relative error of 13%. Also the C2X method presents good adjustment with the same bands ratio, although with a greater error, while the correction C2RCC shows the worst correlation. The relationship between SD (in m) and turbidity (in NTU) has also been obtained, which provides an operational method for estimating turbidity with S2. The relationship for the different reservoirs between SD and chlorophyll-a concentration, suspended solids and dissolved organic matter, is also shown.</p>

Effect of Phosphorous Loadings on Macrophytes Structure and Trophic State of Dam Reservoir on a Small Lowland River (Eastern Poland)

The main objective of the study was to evaluate if macrophytes structure and trophic status of dam reservoir Kraśnik on a small lowland river Wyżnica are determined by phosphorous loadings. Studies were conducted seasonally in May, July and October during the years 2008-2009. Samples were taken at four sites: Site 1 - inflow of the Wyżnica River to pre-dam, Site 2 − pre-dam, Site 3 − dam reservoir and Site 4 - outflow of the Wyżnica River from dam reservoir. Physical and chemical parameters (temperature, Secchi disc depth, dissolved oxygen, pH, conductivity, total suspension, chlorophyll-a, TP and P-PO4) were measured in water samples. Together with water parameters there were estimated biomass of phytoplankton and species composition and biomass of emergent, floating-leaved and submerged macrophytes. Concentrations of TP, chlorophyll-a and Secchi disc depth were used to calculate trophic state index of Carlsson for dam reservoir and its pre-dam. Based on mean water current, mean residence time of water in dam reservoir and concentrations of TP and P-PO4 loadings (g m-2) flowing into dam reservoir with the Wyżnica River were calculated. The results showed visible negative effect of phosphorous loadings on both macrophytes composition and trophic state of the reservoir. The marked changes concerned soft vegetation. High P loadings (7.74 g m-2 of TP and 6.03 g m-2 P-PO4) during the spring of 2008 caused the disappearance of characeans meadows. In 2009, the presence of rigid hornwort (Ceratophyllum demersum L.), the species typical for eutrophic lakes was noted. This unrooted submerged plant uptakes dissolved orthophosphates directly from the water column. Values of Carlsson index (51.4 ≤ TSI ≤ 68.2) indicate the eutrophic state of dam reservoir Kraśnik. During summer season in dam reservoir there were observed algal blooms (biomass of phytoplankton exceed 10 mg WW dm-3) and low water transparency (Secchi disc depth ranged from 0.4 to 0.65 m). During the two-year studies in dam reservoir Kraśnik a high reduction of P loadings, mostly dissolved orthophosphates was observed. Dependently on season, reduction of P-PO4 loadings ranged from 52% (July 2008) up to 91% (May 2009). The reduction of TP was lower and reached values from 15% (May 2008) to 48% (July 2009).

Turbidity in the northern Great Barrier Reef lagoon in the wet season, March 1989

In 1989, a typical wet season was experienced in northern Queensland, with low winds and long calm periods. Turbidity in upper waters of the Great Barrier Reef lagoon broadly had a simple distribution that could be modelled from bottom depth contour values alone, without introducing wind speed or bottom type. In the absence of major storm and cyclone events, this result appears to be general, based on the similarity between March 1989 survey data and Secchi disc climatology. The simple distribution arises because the main turbidity sources are riverine discharges, with little entrainment of bottom sediment into the upper column, except in shallower waters. Fresh, highly turbid riverine influxes are generally confined close inshore, with salinity and Secchi contours parallel to shore, forming cross-shelf gradients. A semi-quantitative relation was found between sea surface colour and Secchi disc depth. Examination of nephelometric turbidity stratification showed that satellite and Secchi data should be more useful for subsurface turbidity inference between Cooktown and Innisfail than in Princess Charlotte Bay, with horizontal and vertical stratifications, respectively, observed in those areas. Highest nephelometric turbidity was seen from Cooktown to Innisfail. Beam attenuation coefficient in oceanic waters outside the reef appeared to be dominated by absorption, with lagoon waters influenced by scattering. A method is suggested to enable approximate transfer of beam attenuation coefficient measured by a transmissometer operating at a single wavelength to beam attenuation coefficient at other wavelengths, using coincident measurements of Secchi disc depths made with filters.

Dependance of phytoplankton chlorophyll on bottom resuspension in Cleveland Bay, northern Queensland

In relatively shallow regions of the lagoon of the central Great Barrier Reef. phytoplankton chlorophyll a concentrations are dependent on intermittent resuspension of bottom sediment by wind-generated waves. This results in a strong inverse correlation between chlorophyll a levels and transparency over a period of time. Bottom resuspension and resettling cause a mide transparency range to occur at any given station. but the mean Secchi disc depth increases linearly with water depth across the lagoon.

Physico-chemical aspects of Chandbill Oxbow lake of Meherpur, Bangladesh

Some physico-chemical parameters of Chandbill Baor, a natural oxbow lake of Meherpur district were studied from September 2006 to August 2007. Air temperature of the study area varied from 18.10 to 32.10°C (x-bar± SE : 27.73 ± 1.22°C). The Water temperature (17.10 to 32.10°C; x-bar± SE: 28.24 ± 1.32°C), Secchi disc depth (8.89 to 53.34 cm; x-bar± SE : 26.19 ± 4.29 cm), water depth (132.08 to 307.34 cm; x-bar± SE : 183.93 ± 13.88 cm); total alkalinity (51.3 to 85.5 mg/l; x-bar± SE : 68.4 ± 3.64 mg/l), total hardness (85.5 to 188.10 mg/l; x-bar± SE : 125.81 ± 9.38 mg/l), dissolved oxygen (5.0 to 12.0 mg/l; x-bar± SE: 8.42 ± 0.62 mg/l), free CO2 (5.0 to 30.0 mg/l; x-bar± SE : 14.58 ± 2.64 mg/l), pH (8.0 to 10; x-bar± SE : 8.54 ± 0.19), ammonia-nitrogen (0.3 to 0.7 mg/l; x-bar± SE: 0.46 ± 0.04 mg/l) in the 'Baor' water were found to be suitable for survival of aquatic fauna. Nitrite-nitrogen was absent in the 'em>Baor' throughout the year. Positive correlations were observed in between air and water temperature (r = 0.978); free CO2 and Secchi depth (r = 0.839), free CO2 and water depth (r = 0.714); water depth and Secchi depth (r = 0.903); total hardness and total alkalinity (r = 0.861) and pH with DO (r = 0.661). However, inverse relations were observed among ammonia-nitrogen with water depth (r = −0.727); ammonia-nitrogen with Secchi disc depth (r = −0.840); dissolved oxygen with free CO2 (r = −0.636); pH with Secchi disc depth (r = −0.581) and pH with free CO2 (r = −0.825). The productivity of the lake was found to be medium and could be improved to higher level. Alkaline pH (8.54 ± 0.19) was supposed to be helpful for proper growth and development of fishes and aquatic organisms. This is the first limnological report from a natural ‘Baor’ (Oxbow lake) of Meherpur district, Bangladesh.Key words: Physico-chemical aspects; Oxbow lake; MeherpurDOI: http://dx.doi.org/10.3329/dujbs.v20i1.8835Dhaka Univ. J. Biol. Sci. 20(1): 31-39, 2011 (January)