Effects of elevated temperature and CO2 on intertidal microphytobenthos

Abstract

Background: Microphytobenthos (MPB) are the main primary producers of many intertidal and shallow subtidal environments. Although these coastal ecosystems are particularly vulnerable to anthropogenic activities, little is known on the effects of climate change variables on the structure and productivity of MPB communities. In this study, the effects of elevated temperature and CO2 on intertidal MPB biomass, species composition and photosynthetic performance were studied using a flow-through experimental life support system.

Results: Elevated temperature had a detrimental effect on MPB biomass and photosynthetic performance under both control and elevated CO2. Furthermore, elevated temperature led to an increase of cyanobacteria and a change in the relative abundance of major benthic diatom species present in the MPB community. The most abundant motile epipelic species Navicula spartinetensis and Gyrosigma acuminatum were in part replaced by tychoplanktonic species (Minidiscus chilensis and Thalassiosira cf. pseudonana) and the motile epipelic Nitzschia cf. aequorea and N. cf. aurariae. Elevated CO2 had a beneficial effect on MPB biomass, but only at the lower temperature. It is possible that elevated CO2 alleviated local depletion of dissolved inorganic carbon resulting from high cell abundance at the sediment photic layer. No significant effect of elevated CO2 was detected on the relative abundance of major groups of microalgae and benthic diatom species.

Conclusions: The interactive effects of elevated temperature and CO2 may have an overall detrimental impact on the structure and productivity of intertidal MPB, and eventually in related ecosystem services.

Figure 1

Microphytobenthos NDVI under control and elevated CO ₂ and temperature. Changes in normalized difference vegetation index (NDVI, mean ± standard error, n = 4) of an intertidal sediment during an 11-day period under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4

Figure 2

Microphytobenthos Chl a under control and elevated CO ₂ and temperature. Chlorophyll a concentration (Chl a, mean ± standard error, n = 4) of an intertidal sediment (0–2 mm) at the beginning of the experiment (T0) and after 11 days under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4

Figure 3

Microphytobenthos F _v / F _m under control and elevated CO ₂ and temperature. Changes in maximum efficiency of photosystem (PS) II (F _v/F _m, mean ± standard error, n = 4) of an intertidal sediment during an 11-day period under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4.

Figure 4

Microphytobenthos RLC parameters under control and elevated CO ₂ and temperature. Changes in relative maximum electron transport rate (rETR _max, A), light utilization coefficient (α, B) and light saturation parameter (E _k, C) (mean ± standard error, n = 4) of an intertidal sediment after 0, 6 and 11 days under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4.

Figure 5

Relative abundance of major groups of microphytobenthos under control and elevated CO ₂ and temperature. Relative abundance (%, mean ± standard error, n = 4) of major groups of microphytobenthos (diatoms, cyanobacteria, euglenophytes and dinoflagelates) of an intertidal sediment (0–2 mm) at the beginning of the experiment (T0) and after 11 days under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4.

Figure 6

CLUSTER analysis of diatom assemblage structure under control and elevated CO ₂ and temperature. Dendrogram for hierarchical clustering using group-average linking of Bray–Curtis similarities of diatom abundance of an intertidal sediment (0–2 mm) at the beginning of the experiment (T0) and after 11 days under different temperatures and pH. T 18°C, pH 8.0: Temperature = 18°C, pH = 8.0; T 24°C, pH 8.0.Temperature = 24°C, pH = 8.0; T 18°C, pH 7.4.Temperature = 18°C, pH = 7.4; T 24°C, pH 7.4: Temperature = 24°C, pH = 7.4. Dashed lines indicate groups of samples not separated (at p < 0.05) by SIMPROF.