Aim

The overall objective of this study is the estimation of the early, mid- and longterm effects of ultraviolet radiation (UVR; 280-400 nm, further divided into UVBR; 280-320 nm and UVAR 320-400 nm) on the succession of benthic primary producers in the presence or absence of grazers. In particular, the impact of ambient (and enhanced) UVR on the succession of micro- and macroalgae will be determined in the rocky intertidal in Potter Cove on King George Island, Antarctica.

Background

Marine macroalgae are the most important primary producers in coastal hard bottom ecosystems. Furthermore, they make an essential contribution to the structural heterogeneity of their habitat, thereby increasing the number of ecological niches available to other organisms. Macroalgal stocks serve as nursery areas and shelter for numerous animals and provide a substrate for epiphytic communities. In this way they are crucial prerequisites for the diversity and stability of coastal ecosystems. Marine microalgae are generally dominated by diatoms. Marine diatoms constitute the basis of the marine food we b and are responsible for 50% of the global marine primary productivity. In Potter Cove, benthic diatoms are of particular interest because the phytoplankton biomass is not sufficient to explain the benthic consumer abundance and it has been hypothesized that microbenthic algae (diatoms) account for the nutrition of the local fauna.

The seasonal depletion of the stratospheric ozone layer and the resulting increase in UVBR reaching the Earth’s surface, particularly over the Antarctic region, is a potential threat to all organisms, including marine macro- and microalgae.

In the study area, UVR penetrated down to a depth of 10-30 m and 1% of the surface UVBR intensity was reached at 9 m depth. Thus, the UVR penetration depth is large enough to cause damaging effects on both tidal and subtidal organisms.

UVR has several damaging effects on biological molecules and metabolic pathways. On the other hand, there are also protective and repair mechanisms. If the damaging effects predominate then growth and reproduction are inhibited. The role of UVBR on the community and ecosystem level is still poorly understood and few long-term experiments using natural communities or model ecosystems have been carried out. Organisms on several taxonomic levels appear to differ widely in their tolerance and in their capacity to adapt to UVR and this may result in changed species compositions. The few existing studies that have considered effects on more than one functional level within natural or seminatural systems have all demonstrated complex responses to UVBR and the need for integrative studies. The results of our field and laboratory experiments will allow us to predict the consequences of UVR, including enhanced UVBR, for the diversity and stability of the algal community.

Field work

Our study will continue over two growth seasons, November 2003 to March 2004 and October 2004 to March 2005. Thus, the experiments will start during the main growth season of Antarctic algae concomitant with sea ice break-up in October and the time of highest UVBR due to the seasonal depletion of the ozone layer in the Antarctic region. The first field experiment has been completed and in addition several laboratory experiments on both micro- and macroalgae are running or have been completed. In this report we focus on the field experiment and very briefly report same results from the microalgal experiments.

Experimental design

32 experimental units with ceramic tiles (photo 2) were installed on an intertidal platform. The number of replicates were four and different filters were used to exclude parts of the solar spectrum. The experimental chambers were further designed to allow or prevent grazing on the algae. PAR = photosynthetic active radiation (400-700 nm) (see below).

Treatments:

PAR+UVAR+UVBR (grazer-no grazer)
PAR+UVAR (grazer-no grazer)
PAR (grazer-no grazer)
controls for filters and cage artefacts

Four samplings took place, the first after 4 weeks, then approximately every 16 to 17 days (depending on weather conditions). One large and one small tile were removed at each sampling occasion. The algae were collected, identified and treated for further chemical analyses. Biomass and composition of micro- and macroalgae were analysed by light microscopy. The biomass data was used to calculate the biodiversity of the communities, employing the Shannon-Weaver index H’, together with the species richness S and the evenness index J’. Photosynthetic activity of microalgae, macroalgal spores or early life stages of macroalgae were investigated at the times of collection by using different models of pulse-amplitude modulated PAM fluorometers (PAM 2000, Xenon PAM, Walz, Germany). Photosynthetic characteristics can be described by measuring the optimum quantum yield (F_v/ F_m), i.e. the ratio of variable to maximum chlorophyll fluorescence after dark adaptation. Furthermore, the harvested material will be analysed for tissue C:N ratios, content and composition of photosynthetic pigments, content and composition of mycosporine-like amino acids (UV absorbing compounds) and for DNA damage. Moreover a few species will also be fixed for electron microscopical examination.

Statistical analysis

Data will be analysed by factorial ANOVA (analysis of variance) including ”grazers” (GRAZERS vs. NO GRAZER) and ”radiation” (PAR, PAR+UVAR, PAR+UVAR+UVBR, Control) as the main effects.

Light measurements

Because light is crucial in our study we used several different light meters. PAR (atmosphere) was measured continuously with a Li-Cor data-logger (LI-1000, Li-Cor, USA), equipped with a flat-head sensor (Ll-190), and UVBR was measured using a 32-channel single-photon counting spectroradiometer developed at the AWI and installed on the roof of the Dallmann Lahoratory. Underwater light measurements were monitored using an underwater spherical quantum sensor (LI 193 SA) and underwater spectra of ambient radiation of the wavelength from 327 to 700 nm were recorded at various depths with a portable spectroradiometer (lngenieurbüro Kruse, Germany).

Microalgal experiments

The objective of this study was to estimate the short-term (hours) impact of UVBR and UVAR respectively on the photosynthetic capacity of a shade adapted benthic diatom community.

Fine-grained sand y sediment was collected from 5 m water depth, divided into an experimental chamber consisting of 12 chambers. Six short-term experiments (12-46 h) were carried out in a temperature and light controlled set-up (photo 4). Treatments were PAR, PAR+UVAR, PAR+UVAR+UVBR, respectively. The experiments were performed with intact diatom mats or diatom slurries. The algae were exposed to UVR for 6 hours followed by PAR only for 12-20 hours and a period of darkness (µmol photans m^-2 s^-1) for another 10-20 hours.

The effects of UVR on photosynthetic activity was determined by measuring the emission of variable chlorophyll fluorescence by use of PAM 2000 and the WATER-PAM (Walz, Germany). Maximum quantum yield of photosynthesis was measured in light exposed diatom mats and in diatom slurries after dark adaptation by determination of the ratio of F_v/ F_m. Any decrease in F_v/ F_m reflects photoinhibition or even photodamage of the photosynthetic apparatus.

First results

Field experiment

In all cases the non-UV treatment showed a higher diversity than the ones with UVAR or UVBR present. Significant UVR effects on diversity were found for the first and the last sampling. No effect of UVR on the biomass of the samples was observed, whereas there was a strong consumer effect (mostly due to gastropods).

In general, UVBR had more pronounced negative effect- (mostly due to gastropods). The results show that small stages of macroalgae are particularly sensitive to UVR (diversity and density of germlings decreased on tiles with UVR). Similarly, additional laboratory experiments show that spores from macroalgae from the intertidal are highly susceptible to UVR.

Microalgal experiments

In all experiments, the maximum quantum yield of photosynthesis decreased significantly after 2-6 hours exposure to UVBR or UVAR, respectively (figure 2). The following exposure to PAR alone did not result in any recovery in UVR treated samples. However, after 4-12 hours in darkness the yield was restored to original values.

The result show that UVR clearly has a damaging effect in our short-term experiments but this effect was reversed after a period in darkness (low light). Future studies have to show how the algae react in long-term experiments.

Where are we going from here?

This season we will use the same field setup and hopefully we will be able to start the experiment one month earlier.  The laboratory experiments will be refined and enlarged. The project has already generated a large number of data that will be analysed and interpreted. The species analyses will take 1-2 years to complete. Results from some of the microalgal experiments were presented at SCAR open science meeting, Bremerhaven, July 2004, and are now compiled for publication.