Majestic undersea meadows and their hungry residents

Like most biology enthusiasts, I love an excuse to go into the field. So when given complete freedom to investigate any invertebrate question we wanted for our final project, I didn’t have to think twice before choosing to study invertebrates in seagrass meadows: a severely underappreciated ecosystem type.

Yes I know- seagrass sounds like just about the most uncharismatic thing there is. Grass isn’t very interesting at all…

boring grass

Figure 1. Boring land grass. Image: 1234RF Stock Images

… so grass under the sea probably isn’t that much more interesting, right? Perhaps these images might change your mind…


Some words that come to my mind from looking at these ones include “tranquil”, “majestic” and “lush”, which is a far cry from the average person’s conceptions of grass. So what’s the big deal with seagrass, and more importantly, what on Earth do invertebrates have to do with them?

Unlike seaweeds, seagrasses are marine angiosperms: land plants that have colonized the ocean. Different species of seagrasses are found all over the world with the exception of the high poles. As is apparent from the photos above, seagrass meadows are highly productive; the grasses harness energy from the sun, creating far more biomass per unit area than microscopic phytoplankton do in pelagic (open) waters. They also stabilize the sediment, and provide habitat for a diversity of life on the same order of magnitude as some coral reefs, from tiny invertebrates to juveniles of commercially important fish. Invertebrate herbivores (also known as grazers) living in seagrass meadows have one very important job- to eat! Grazers play the critical role of transferring all the nutritious food generated in seagrass meadows to animals higher up in the food web like fish. Juvenile fish that use seagrass meadows as nursery habitat eat lots and gain weight, and eventually they transport this biomass to pelagic waters when they are large enough to leave. In other words, seagrass meadows are important for creating biomass that ends up in open ocean, and grazers are a key player in this process.

meep

Snorkelling at the field site in Tsawwassen to collect seagrass-associated invertebrates for my experiment

Lucky for me, there is a seagrass meadow not too far away where I was able to get my field fix and collect my experimental organisms. The low tides that week were quite high so I ended up having to snorkel to catch them- no complaints there (aside from not being able to feel my face in the 6°C water). I also happened to visit in the middle of an algae bloom; the seagrass was looking pretty scummy from algae growing on the blades.

The most abundant grazers at the field site were the green eelgrass isopod Idotea resecata and the eelgrass limpet Lottia parallella. The results of my experiment showed that both actually prefer eating seagrass over algae, which may make seagrass at the site more vulnerable to decline from being strangled by the algae (More details here). Seagrass meadows are experiencing disastrous losses all around the world; since the 1940s, we have lost seagrass meadows on an order of magnitude of 10 000 km2 1. Despite this, they are not getting nearly as much attention from the media and general public as their charismatic counterparts, coral reefs. Along with conducting research to further understand factors causing their decline, creating public awareness and appreciation of their importance is crucial.

mesocosm

Who needs a tropical reef fish aquarium when you can have a seagrass mesocosm instead?

Source

  1. Orth, R. et al. A Global Crisis for Seagrass Ecosystems. Bioscience 56, 987–996 (2006).
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