Light Requirements for Growth and Survival of Eelgrass (Zostera marina L.) in Pacific Northwest (USA) Estuaries

Abstract

We developed light requirements for eelgrass in the Pacific Northwest, USA, to evaluate the effects of shortand long-term reductions in irradiance reaching eelgrass, especially related to turbidity and overwater structures. Photosynthesis-irradiance experiments and depth distribution field studies indicated that eelgrass productivity was maximum at a photosynthetic photon flux density (PPFD) of about 350-550 μmol quanta m −2 s −1 . Winter plants had approximately threefold greater net apparent primary productivity rate at the same irradiance as summer plants.Growth studies using artificial shading as well as field monitoring of light and eelgrass growth indicated that longterm survival required at least 3 mol quanta m −2 day −1 on average during spring and summer (i.e., May-September), and that growth was saturated above about 7 mol quanta m −2 day −1 . We conclude that non-light-limited growth of eelgrass in the Pacific Northwest requires an average of at least 7 mol quanta m −2 day −1 during spring and summer and that long-term survival requires a minimum average of 3 mol quanta m −2 day −1 .

Annual and perennial life history strategies of Zostera marina populations under different light regimes

“…The survival of seagrass is closely related to underwater PFD and duration of H comp (Dennison 1987, Lee et al 2007b). Non-lightlimited growth of Z. marina in the Pacific Northwest reportedly requires at least 7 mol photons m −2 d −1 , whereas long-term survival requires a minimum average of 3 mol photons m −2 d −1 (Thom et al 2008). Seedlings of Z. marina require a minimum of 8 mol photons m −2 d −1 on average for rhizome and leaf growth during their first summer (Bintz & Nixon 2001).…”

“…The duration of PFDs above light saturation points (H sat ) has also been used to predict carbon balance and seagrass production (Dennison & Alberte 1982, 1985, Zimmerman et al 1994. Therefore, reduction in underwater light intensity and/or duration is one of the main environmental stresses leading to seagrass declines and affecting distribution patterns (Cabello-Pasini et al 2002, Holmer & Laursen 2002, Thom et al 2008, Biber et al 2009). …”

Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest, USA

“…Turbidity is generally higher (20-30 NTU) with more frequent high pulse events associated with storm events at Danger than at the other sites (Rumrill 2006). Elevated turbidity levels can be related to decreased water column light levels, which may affect eelgrass photosynthetic capabilities (Moore et al 1997;Thom et al 2008;Zimmerman et al 1995). Because eelgrass at this site (Danger) may have pre-existing physiological stress associated with low light levels, additional reduction of light by the macroalgae addition treatment could have had a larger impact on plants.…”

Effect of oyster aquaculture on seagrass Zostera marina at the estuarine landscape scale in Willapa Bay, Washington (USA)

“…Models with ele vation, salinity, and distance to estuary mouth as covariates and stress and distance to nearest channel as independent effects were found to produce the best fit. Some of these predictors, such as tidal elevation and distance to estuary mouth, which were the most significant, were likely only proxies for mechanistic variables such as light and dessication, which have been shown to limit the lower and upper distribution of these plants in estuaries re spectively (Dennison 1987, Koch 2001, Boese et al 2005, Thom et al 2008), but for which we did not have spatial data. While the predictor variables used in the model are also auto-correlated, our goal here was to compare the distri bution of Z. marina within and outside oyster aquaculture, so the assumption of independence is less important and perhaps interaction terms be tween these factors and aquaculture are of primary interest.…”