Zostera | MARINe

Zostera (Eelgrass)

Zostera marina (Linnaeus)

Last updated November, 2015

Kingdom Plantae, phylum Tracheophyta, class Monocots, order Alismatales, family Zosteraceae

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Description

Eelgrass is an angiosperm with true leaves, stems, and rootstocks; not an alga.

Z. marina leaf blades are characteristically flat and wide (2-12 mm) and can reach up to 3 meters in length (Mondragon and Mondragon, 2003) although morphology is variable and depends on environmental factors such as substrate type (Short, 1983), depth (Lee et al., 2000), temperature (Moore et al., 1996), and light and nutrient availability (Short, 1983). Bright green leaves arise from creeping rhizomes with many hair-like roots (Lindeberg and Lindstrom, 2010).

Habitat and Geographic Range

Z. marina can be found at a depth of +2m to -12m (Short et al., 1993) from Alaska to Baja California (Wyllie-Echiverria and Ackermann, 2001). Eelgrass is commonly found on mud and sand in protected bays and estuaries (Mondragon and Mondragon, 2003).

Synonyms

None

Similar species

There are several other species of Zostera that resemble Z. marina. An example of one of these is Zostera noltii, a nonnative species of eelgrass that was introduced from Europe and is present in Washington and southern British Columbia (Mondragon and Mondragon, 2003). Zostera japonica is another nonnative eelgrass that is from Japan and is believed to have been introduced via oyster culture (Posey, 1988). Both of the aforementioned species are generally smaller and narrower than Z. marina. However, Z. asiatica may also sometimes coincide with Z. marina and is more similarly sized (Moore and Short in Larkum et al., 2006).

Natural History

Eelgrass is one of the earliest studied seagrasses (Petersen 1890, 1918). Zostera likely originated in the Pacific between 8 and 20 million years ago (Olsen et al., 2004). It is usually perennial and monoecious, meaning individual plants can produce both male and female flowers. However, only a fraction of seeds that germinate survive to adulthood (Moore and Short in Larkum et al., 2006). Eelgrass is one of the most productive of marine primary producers (Duarte et al., 2005) and it provides food and shelter, including nursery habitat (Heck et al. 2003), for many marine species including some of economic importance (Duarte et al., 2005). Many birds also depend on Zostera beds for foraging and stop-over locations during migration (Wyllie-Echeverria and Ackerman, 2002; Matsunaga, 2000).

Eelgrass beds are important for filtering water by trapping sediments (Heiss et al. 2000) and for dampening waves and currents (Koch and Verduin, 2001). Zostera can also take up contaminants from the water column (Hoven et al., 1999) and it plays an important role in nutrient cycling (Hansen et al. 2000). Eelgrass beds are also important carbon sinks that can help reduce Earth’s carbon dioxide levels (Duarte et al., 2013).

Several factors threaten eelgrass fitness. Zostera can undergo wasting disease caused by a marine slime mold-like protist, Labyrinthula zosterae (Muehlstein et al. 1991). This disease is spread via leaf to leaf contact and results in brown dots and streaks on leaves (Muehlstein, 1989), then eventual death due to decreased photosynthesis (Ralph and Short, 2002). Zostera is especially sensitive to increased human settlement which leads to increased nutrient and sediment runoff which in turn reduces light availability (Moore et al., 1997) and increases smothering due to algal overgrowth (Bowen and Valiela, 2001). More specifically, dredging, filling, marina development, boat activity, fishing practices, and hardening of the shoreline are all human-based activities that are leading to the destruction of eelgrass beds (Kendrick et al., 2000). Because of these sensitivities, Zostera can be used as an indicator of environmental health conditions (Short et al., 1993).

Monitoring of eelgrass is extremely important because some beds that are declining have yet to be documented. Although monitoring requires substantial time, effort, and money, it produces important information that can be used to reverse declines and increase protection and restoration of eelgrass beds that are declining and might otherwise go unnoticed. Generally, the most effective method of eelgrass restoration is to increase water clarity, however, this solution is often costly and difficult due to overlapping human use of coastal areas (Moore and Short, in Larkum et al., 2006). An effort to increase public awareness should be made to highlight the important role that eelgrass plays in the ecology and economically important fisheries of coastal oceans (Moore and Short, 2006 in Larkum et al., 2006).