Photosynthetic responses of two temperate seagrasses across a water quality gradient using chlorophyll fluorescence

Abstract

Photosynthetic characteristics of seagrasses measured by pulse amplitude modulation (PAM) fluorometry have demonstrated acute responses of seagrasses to short-term doses of high light irradiance, UV radiation, osmotic stress and desiccation. In this work, we use PAM fluorometry to examine photosynthetic rates of the seagrasses Zostera tasmanica and Zostera capricorni across a light gradient within a large temperate marine embayment in Westernport, southern Australia. Three sites were studied across the bay representing a gradient in water quality, with low light conditions at Charing Cross, intermediate light conditions at Crib Point and high light availability at Newhaven. In the field electron transport rates (ETRs) were measured both as rapid light curves (RLCs) by in situ yield measurements under ambient daylight. Z. capricorni showed a high photosynthetic activity compared with Z. tasmanica, a response consistent with the adaptation of Z. capricorni to high light conditions. The low midday photosynthetic rate (ETR max) and photosynthetic efficiency ( α) of both species at Charing Cross where photon fluxes were low suggests a photoadaptive response. Diurnal responses of ETR, photosynthetic efficiency ( α) and I k for both seagrass species were consistent with diurnal responses reported for other seagrass species in Australia. Low early morning photochemical quenching (qP) of Z. tasmanica and Z. capricorni at Charing Cross (low light site) also may be a function of reduced ETR and photosynthetic efficiency. A key finding was the persistent low dark-adapted maximum quantum yield ( F v/ F m) of Z. tasmanica at Charing Cross in both winter and summer due to a decrease in F m relative to F o. Such a finding suggests photosynthetic downregulation and closure of photosystem II (PSII) reaction centres possibly in response to light deprivation. The response suggests seagrasses at Charing Cross showed physiological signs of stress characterised by low quantum yields and low rates of electron transport. Low photosynthetic rates may in part explain the twofold to threefold lower leaf biomass of Z. tasmanica shoots at this site compared with the other two sites.