Light and Seagrasses

Unlike land plants, seagrasses live in an aqueous environment that is basically a “soup” of suspended sediments, organics, and microorganisms. This affects the transparency of the water and therefore the amount of light that can reach the leaves of seagrasses. The reduction in the intensity of light over a given distance is known as light attenuation. Light attenuation is a major factor governing the survival and distribution of seagrasses because compared to other types of marine SAV (submerged aquatic vegetation) seagrasses require a significantly greater amount of light in order to survive. Eelgrass needs 15-22% of the light available at the water’s surface to reach the leaves compared to only about 1% for algae (including seaweeds) (Kemp et al 2004; Zimmerman et al. 1997). It can take as little as 20 days of reduced light availability to cause decline in Z. marina (Moore et al. 1997). The main contributors to light attenuation in the water column are turbidity, phytoplankton, and dissolved organic matter. Light attenuation can also occur at the surface of the leaf due to growth of algae/epiphytes.

Light is the main factor that determines the depth limit for seagrasses.  To cope with limited light, eelgrass tends to have wider and longer leaves with increasing depth as well as less below ground biomass (roots and rhizomes).  Decreased light also leads to a reduction in shoot density, number of leaves per shoot, growth rate, and overall flowering success. On the other hand, eelgrass under high light conditions tends to have smaller shoots but higher densities and productivity.

Seagrass biologists use a variety of instruments and methods to measure different aspects of light attenuation: 

• The clarity of the water can be measured using a Secchi disk. This instrument is very simple, consisting of a black and white disc (see photo) that is lowered into the water column until it is no longer visible. It doesn’t give an actual measurement of light penetration, but the “secchi depth” is very useful for comparative purposes in conjunction with other water quality tests.
• Turbidity is measured in several ways. A nephelometer (turbidity meter) is an instrument that measures the amount of light reflected by particles in the water, expressed in NTU’s (Nephelometric Turbidity Units).  Measuring the total suspended solids (TSS) is another way to measure turbidity. Though more labor intensive in that it requires a lab to measure the dry weight of particulates in a sample (usually expressed as mg/l), it is more accurate in that it includes larger particles that settle too fast for a nephelometer to read.
•  Light loggers can take light readings over a period of time to determine an average amount of light reaching a particular area. The amount of surface light compared to the amount of light reaching the bottom will determine how much light is attenuated by the water column.
• Concentrations of Chlorophyll A are measured to determine the concentration of phytoplankton in the water column. This test is used to indirectly determine if waters are nutrient loaded.
• An instrument known as a PAM (pulse amplitude modulated) fluorometer measures specific aspects of photosynthesis and can indicate if a plant. A Diving PAM enables researchers to take these measurements “in situ”, or where the seagrass occurs, in order to get the most accurate measurements.