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 contribute essentially 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, as well as providing 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 web 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 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 10 to 30 m depth and 1% of the surface UVBR intensity was reached at around 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 (figure 2). On the other hand there are also protective and repair mechanisms. If the damaging effect predominates then growth and reproduction are inhibited. The role of UVBR at the community and ecosystem level is still poorly understood and few long-term experiments using natural communities or model ecosystems have been carried out. On several taxonomic levels organisms appear to differ widely in their tolerance and in their capacity to adapt to UVR, which 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.

Fieldwork

Our studies have continued over two growth seasons, December 2003 to March 2004 and November 2004 to March 2005. Thus the experiments started during the main growth season of Antarctic algae concomitant to the time of 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. In addition to the main field experiments, several laboratory experiments on both micro and macroalgae have been completed.

Experimental design

32 experimental units with ceramic tiles (figure 3) were installed on an intertidal platform. The number of replicates was 4, plus control cages for filter and cage effects. 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 (open vs. closed cages).

Four samplings took place, the first after 4–6 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 (Fv/Fm), i.e. the ratio of variable to maximum chlorophyll fluorescence after dark adaptation. A few species were also fixed for electron microscopic examination.

Statistical analysis

Data were 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 (LI-190). UVBR was measured using a 32-channel single-photon counting spectroradiometer developed at the AWI and installed on the roof of the Dallmann Laboratory. Underwater spectra of ambient radiation of the wavelength 280–700 nm were recorded at various depths using a spectroradiometer (Isitec, Germany).

Microalgal experiments

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

Fine-grained sandy sediment was collected from 5 m water depth and divided into experimental chambers in the laboratory. In total 22 experiments were carried out. Treatments were PAR, PAR+UVAR and PAR+UVAR+UVBR, respectively. The experiments were performed with intact diatom mats or diatom suspensions. The different treatments are shown in Table 1.

The effects of UVR on photosynthetic activity were determined by measuring the emission of variable chlorophyll fluorescence by use of PAM 2100 and the water-PAM (see above). Maximum quantum yield of photosynthesis was measured in light exposed diatom mats and in diatom suspensions after dark adaptation by determination of the ratio of Fv/Fm. Any decrease in Fv/Fm reflects photoinhibition or even photodamage of the photosynthetic apparatus.

Some results

Field experiment

• UVR had a negative impact on a number of macroalgal species, individuals and diversity (figure 4a).
• Green algal germlings were more affected by UVR during early stages of succession.
• Red algal germlings were more sensitive to UVAR than to UVBR, showing that
• UVAR had a large impact on community structure.
• Effects can change during succession but are different for different species. Thus UVR could lead to shifts in the distribution and abundance of macroalgal species in the intertidal.
  

  The macroalgal diversity after 6 and 15 weeks in relation to the different UV treatments is shown. PAB means exposure to PAR+UVA+UVB, PA exposure to PAR+UVA and P means exposure to PAR only. The letters a, ab, and b denote significant differences between treatments (p<0.05). After 6 weeks no significant differences between the treatments were found. After 15 weeks the diversity for the PAB treatment decreased, whereas it increased for the P treatment.

  The light effect on the maximum quantum yield of photosynthesis (Fv / Fm) for benthic diatoms after 4 h of light exposure and different recovery times (rec) where they have been exposed to PAR only. PAB means exposure to PAR+UVA+UVB, PA exposure to PAR+UVA and P means exposure to PAR only. “Undist” is the untreated diatoms and “init” means initial values before the treatment started.

  The light effect on the maximum quantum yield of photosynthesis (Fv / Fm) for benthic diatoms after 8 h of light exposure and different recovery times (rec) where they have been exposed to PAR only. PAB means exposure to PAR+UVA+UVB, PA exposure to PAR+UVA and P means exposure to PAR only. “Undist” is the untreated diatoms and “init” means initial values before the treatment started. Microalgal experiments

The results show that UVR clearly had a damaging effect in both our long-term and short-term experiments, but this effect was reversed after a period in low light (figure 4c). Benthic diatoms can cope with UVB intensities up to at least 32 times higher than what can be expected at 5 m depth in Potter Cove. Furthermore, in terms of Fv/Fm, response to the light intensity and recovery were both measurable within 10 minutes (picture 2). Future studies have to show how the microalgae react in longterm experiments with ambient radiation.

Where are we going from here?

The project has generated a large number of data that still need to be analysed and interpreted. The species analyses will take 1–2 years to complete. Results from both field and laboratory experiments have been presented on the SCAR Open Science Meeting, Bremerhaven, Germany in July 2004, and at the International Phycological Congress, Durban, South Africa in August 2005.