The effects of ultraviolet radiation on the planktonic community of a shallow, eutrophic estuary: results of mesocosm experiments

Abstract

This paper describes the results of pelagic mesocosm experiments designed to test the effects of enhanced and reduced ultraviolet radiation (UV) on the planktonic community of a Baltic Sea estuary. The Darss-Zingst estuary consists of a series of brackish lagoons with high concentrations of chlorophyll and dissolved organic matter. The shallow depth of the estuary ensures that organisms in the water are regularly exposed to high levels of photosynthetically active radiation (PAR) and UV. During the summer of 1995 and 1996, four 1-m 3 mesocosms were filled with water from the mid-point of the estuary. Each compartment was equipped with a pump to simulate natural rates of windinduced vertical mixing. The mesocosms were hung in the estuary from a floating raft and were shielded from above by filters to give the spectral treatments PAR only, PAR+UV-A, and PAR+UV-A+UV-B. Enhanced levels of UV-B, i.e. twice that of midday sunlight, were provided in a further treatment by artificial sunlamps. Experiments were conducted for periods of 3-14 days. No significant effects of enhanced or reduced UV-B were observed on chlorophyll a concentrations or photosynthetic performance, although the PAR-only treatment did show higher final chlorophyll concentrations in two of the trials. Phytoplankton pigment composition was measured by in vivo absorption and fluorescence excitation spectra, and was similar in all mesocosm treatments indicating that there were no major differences in functional group composition. Bacterial secondary production rates as measured by thymidine incorporation increased with time in all mesocosms, probably due to enhanced production of phytoplankton exudate. There was evidence for a small depression of secondary production by enhanced UV-B, but only on certain days. Microzooplankton generally increased in all mesocosms to population densities higher than those observed in the estuary, and tended to reach higher final values in the mesocosms exposed to UV. It is concluded that vertical mixing which reduces the residence time of planktonic organisms in the surface layers, and high concentrations of chromophoric, dissolved organic matter, which greatly reduce the penetration of UV-B, combined to protect the planktonic community from UV-B damage.

Variable phytoplankton response to enhanced UV-B and nitrate addition in mesocosm experiments at three latitudes (Canada, Brazil and Argentina)

“…This lack of effect of enhanced UV-B for pigments or cell abundance has been observed frequently in studies that use stirred mesocosms (e.g. Keller et al 1997, Wängberg et al 1999, Forster & Schubert 2001, and probably reflects a moderating influence of mixing when repair is present (Helbling et al 1992, Neale et al 1998, Fouilland et al 2003. The presence of at least one such repair mechanism (D1 turnover) was ascertained in our 3 sites (Bouchard et al 2005).…”

Long-Term UVR Effects Upon Phytoplankton Natural Communities of Patagonian Coastal Waters

“…Some outdoor mesocosms studies showed little UVR effects on chlorophyll a (Chl a) content (Halac et al 1997;Wängberg et al 1999;Whitehead et al 2000). On the other hand, Forster et al (2001) working with phytoplankton of the Darss-Zingst estuary in the Baltic Sea found higher Chl a concentration in samples that did not receive UVR, as also observed in the experiments www.intechopen.com Long-Term UVR Effects Upon Phytoplankton Natural Communities of Patagonian Coastal Waters 231 carried out by Keller et al (1997) with communities off Narragansett Bay. At the community level, UVR impact is frequently translated onto changes in the taxonomic composition towards more tolerant species or changes in size distribution.…”

“…Indirect effects of UVR such as the breakdown (photolysis / photodegradation) of dissolved organic matter (DOM) (Osburn and Morris 2003) can be either beneficial for organisms by means of increasing nutrient availability, or detrimental, by increasing water transparency so cells receive more irradiance (Cooke et al 2006). It is also known that the interaction of UVR and other factors -e.g., nutrient availability, vertical mixing, changes in temperature, supersaturating oxygen concentrations or high pH values, may strongly condition the observed results (Forster and Schubert 2001;Neale et al 2003;Beardall et al 2009 and references therein) as compared to those obtained considering only UVR. This is due to the synergic or antagonistic nature of the interactions between UVR and those other factors (Dunne 2010).…”

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“…Mesocosms differ from microcosms in their fundamental concepts. The arrangement of a mesocosm allows assay control, without the mentioned limitations and confounding factors from microcosms, and has been used by many authors (Davies et al, 1995;Forster and Schubert, 2001;Moriñigo et al, 1990;Pommepuy, et al, 1996b;Rhodes, 1988;1990). However, mesocosms cannot provide full access to real aquatic ecosystems, promoting both bacterial species' selective growth and grazing, distorting the community structure (Beardsley et al, 2003).…”

“…However, mesocosms cannot provide full access to real aquatic ecosystems, promoting both bacterial species' selective growth and grazing, distorting the community structure (Beardsley et al, 2003). For example, Forster and Schubert (2001) used a robust 1-m 3 mesocosm to test the effects of enhanced and reduced ultraviolet radiation (UV) on the planktonic community. These authors encountered smaller zooplankton population densities (rotifers and Copepods) that increased rapidly to populations that were denser than those observed in the estuary, probably due to the absence of predators like fish and larger zooplankton.…”