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EFFECT OF AMBIENT LIGHT, AERIAL EXPOSURE, AND SEASON ON EELGRASS (ZOSTERA MARINA) METRICS IN A NORTHEAST PACIFIC (USA) ESTUARY
Citation:
Boese, B L., B. D. Robbins, AND G B. Thursby. EFFECT OF AMBIENT LIGHT, AERIAL EXPOSURE, AND SEASON ON EELGRASS (ZOSTERA MARINA) METRICS IN A NORTHEAST PACIFIC (USA) ESTUARY. BOTANICA MARINA. Walter de Gruyter GmbH, Berlin, Germany, 48:274-283, (2005).
Impact/Purpose:
To determine major factors limiting Z. marina in the upper intertidal
Description:
Although light is the principal factor controlling the lower depth limit of seagrasses, little attention has been given to how reduced winter lighting may affect intertidal plants. In the present study intertidal light intensity, temperature, and aerial exposure were measured over a two year period using continuously recording sensors in a Northeast Pacific estuary eelgrass (Zostera marina) meadow (Yaquina Bay, Newport, OR, USA). Light and temperature sensors were placed along a tidal gradient that ranged from the approximate middle of the permanent eelgrass bed to above the upper intertidal margin for these plants. The duration of aerial exposure was estimated from tidal data collected in Yaquina Bay. Mean daily hours of saturating irradiance (Hsat) were estimated using planar illuminance and eelgrass P vs. I curves obtained from PAM fluorometry. Plant growth (leaf elongation), canopy height, leaf width, and leaves per shoot were measured across tidal gradients at six intervals during the course of the study. P vs. I results indicated that subtidal eelgrass perennials were shade-adapted with intertidal plants appearing to become progressively less shade-adapted with increasing tidal elevation. Photosynthetic pigments in these perennials appeared to become saturated at ~ 64 moles photons m-2 s-1. Hsat values tended to increase with increasing tidal position. During the second winter of the study mean daily Hsat values from low in the intertidal were within the range of values below which the plant must use rhizome reserves to maintain carbon balance. Temperature values across the tidal gradient were greater in summer and lower in winter from low to high tidal elevations, however, these differences appeared not to be physiologically significant (<1 C). Plant growth rates were lower in the winter than summer and high intertidal plants grew at slower rates than lower intertidal plants. When growth rates were normalized to total above-ground biomass, higher intertidal plants tended to have a greater portion of their spring and summer biomass associated with new growth, suggesting that older leaf portions were being lost at a faster rate during the growing season. This was not observed in plants measured during the winter. Overall, the results suggested that winter irradiance was not a limiting factor for Z. marina in the upper intertidal. Seasonal temperatures were highly correlated with growth rates. Many of the upper intertidal plants collected during the spring and summer showed evidence of dessication damage which was consistent with the seasonal differences in normalized growth rates. It was concluded that the major factor limiting Z. marina in the upper intertidal was desiccation stress which was acute and episodic rather than chronic in nature.