Seasonality affects macroalgal community response to increases in pCO2

Abstract

Ocean acidification is expected to alter marine systems, but there is uncertainty about its effects due to the logistical difficulties of testing its large-scale and long-term effects. Responses of biological communities to increases in carbon dioxide can be assessed at CO2 seeps that cause chronic exposure to lower seawater pH over localised areas of seabed. Shifts in macroalgal communities have been described at temperate and tropical pCO2 seeps, but temporal and spatial replication of these observations is needed to strengthen confidence our predictions, especially because very few studies have been replicated between seasons. Here we describe the seawater chemistry and seasonal variability of macroalgal communities at CO2 seeps off Methana (Aegean Sea). Monitoring from 2011 to 2013 showed that seawater pH decreased to levels predicted for the end of this century at the seep site with no confounding gradients in Total Alkalinity, salinity, temperature or wave exposure. Most nutrient levels were similar along the pH gradient; silicate increased significantly with decreasing pH, but it was not limiting for algal growth at all sites. Metal concentrations in seaweed tissues varied between sites but did not consistently increase with pCO2. Our data on the flora are consistent with results from laboratory experiments and observations at Mediterranean CO2 seep sites in that benthic communities decreased in calcifying algal cover and increased in brown algal cover with increasing pCO2. This differs from the typical macroalgal community response to stress, which is a decrease in perennial brown algae and proliferation of opportunistic green algae. Cystoseira corniculata was more abundant in autumn and Sargassum vulgare in spring, whereas the articulated coralline alga Jania rubens was more abundant at reference sites in autumn. Diversity decreased with increasing CO2 regardless of season. Our results show that benthic community responses to ocean acidification are strongly affected by season.

Figure 1. Long-term monthly average day length (hours), rainfall (mm), air temperature (T, °C) and Sea Surface Temperature (°C) for the Saronikos Gulf.

SST data are from the World Ocean Atlas 2013 (NOAA), all other data from the World Meteorological Organisation.

Figure 2. Study sites (points), Loutra baths (*) and area where pH was more variable than at reference sites (light grey).

Geographical data downloaded from OpenStreetMap and modified using GNU Image Manipulation Program 2.8.

Figure 3. Typical appearance of benthic communities at SEEP (left) and 200 E (right) sites at 0.5 m depth in May 2012 with CO₂ bubbles seeping from the sea floor (arrow).

Brown algae (e.g. Dictyota sp.) are dominant near the seeps; crustose coralline algae (CCA) become dominant as CO₂ levels decrease.

Figure 4. Variability in pH at the five study sites off Methana between September 2011 and September 2013.

Horizontal line  =  median, vertical boxes  = 25th and 75th percentiles, whiskers  =  min/max values if smaller than 1.5 times the inter-quartile range and dots  =  outliers.

Figure 5. Shannon diversity (mean H' SE) of macroalgal communities at high, intermediate and reference CO₂ in Methana in May and September 2012.

Figure 6. Mean percentage cover (±SE) of canopy-forming algae (grey) and calcifying algae (black) in May (a) and September (b) at high (n = 6), intermediate (n = 14) and reference (n = 14) CO₂ conditions off Methana.

Different letters indicate significant differences between groups.

Figure 7. Mean percentage cover (±SE) of dominant macroalgal species in May (a) and September (b) at high (n = 6), intermediate (n = 14) and reference (n = 14) levels of CO₂ in Methana.

Round points represent canopy-forming species (S. vulgare dark grey, C. corniculata light grey), rhomboids represent calcifying species (CCA black, J. rubens grey). Different letters and numbers indicate significant differences between groups.