By Victoria Parsons
Around the world, Tampa Bay is earning accolades as one of the few – if not the only estuaries where seagrasses are rebounding even as populations in adjacent watersheds boom.
Florida’s largest open-water estuary, Tampa Bay is recognized as the comeback kid who battled back from the brink, thanks to the grit and determination of a small but determined group of people and a larger community that recognized its value just in time.
Seagrass beds, the cornerstone of life in an estuary and an important indicator species, now cover more acreage in Tampa Bay than they have at any time since the 1950s. With nearly 30,000 acres of seagrasses growing in the bay, scientists note that the pace of recovery has quickened over the last decade, up to about 510 acres per year, compared to about 427 per year prior to 1999.
But underneath the surface, scientists and bay managers are taking a deeper look at seagrasses and seagrass management to reveal a more complex picture:
- Scientists are re-assessing targets for nitrogen loading and water clarity in some bay segments where seagrasses have failed to grow even though water quality has improved dramatically.
- Some scientists are calling for increased focus on the species of seagrasses growing in the bay. Seagrass typically follows a natural sequence from more ephemeral Shoal grass to Turtle and Manatee grass that is more resilient, but current sampling methods are not precise enough to track the changes over time. That raises the question of how healthy seagrasses really are, if mature beds are being replaced with species that are less resilient.
- That potential trend toward more transient species like Shoal grass may mask other downward movements, particularly in areas that are being damaged by prop scars. Those areas are often covered in mature Turtle grasses but areas scarred by boat propellers are being repopulated with Shoal grass that is likely to die back if conditions change.
- Management actions focused on water quality have been extremely effective over the years, but some scientists are calling for an increased emphasis on physical impacts, such as those caused by boat propellers.
If seagrasses continue to rebound at the rapid pace seen over the past 10 years, it will be 2020 or 2025 before the Tampa Bay Estuary Program meets its 38,000-acre goal that reflects 1950s coverage. If the rate of recovery reverts to the average pace, it could be 2045 before the goal set in 1995 is realized.
This issue, Bay Soundings takes a closer look at seagrasses — the good, the bad and the rarely discussed – through the eyes of scientists, advocates and seasoned observers.
Seagrasses dubbed “Nurseries of the Sea”
Often called the “nursery of the sea,” a healthy seagrass meadow is more productive than a fertilized corn field. Seagrasses provide food and shelter for an amazing number of creatures, ranging from microscopic epiphytes that live on leaf blades to the larva of nearly 90% of valuable fish and shellfish, and are a food source for sea turtles and manatees.
They also form the basis of a complex underwater food web, with dozens of species of algae living on their leaf blades and within their root mass. That concentrated food source attracts small marine animals including larval fish and shellfish that feed on the algae and seagrass detritus. Larger predatory fish are attracted by the smaller fish. In fact, some studies indicate that up to 100 times more animals inhabit seagrass meadows than adjacent sandy bottom.
Seagrasses are actually more related to lilies than grasses. They evolved from land plants dating back to the time when dinosaurs roamed the earth. Seagrasses flower underwater, releasing pollen and developing viable seeds that colonize new underwater meadows when conditions are right. Seagrasses also spread across the bay bottom with rhizomes or underground roots that sprout new growth at nodes, much like land-lubber grass in lawns.
As the descendants of land plants, seagrasses require light to survive and they die back if water clouded with algae or other substances prevents light from reaching them on the bay bottom.
Although there are 60 species of seagrasses found in shallow waters around the world, six species are found in Tampa Bay with three species dominating local ecosystems:
Shoal Grass, or Halodule wrightii, occurs in the shallower portions of the bay, land-ward of the larger seagrass species, Turtle grass and Manatee grass. It is the first to colonize disturbed areas, typically followed in a natural succession to Turtle grass and Manatee grass over time. In some locations, it is considered an “ephemeral” seagrass because it comes and goes from year to year. In other locations it is a perennial species, lasting as a seagrass meadow in shallow water from year to year.
It tolerates a wide range of salinities from river mouths to Lower Tampa Bay where it may be exposed to high levels of wave energy. Shoal grass grows to about six inches tall and has very shallow roots. Its flat, narrow blades have notched tips.
Manatee Grass, or Syringodium filiforme, is generally found growing with other species of seagrasses or alone in small patches, usually in calmer, inland sections of the bay. Its blades may grow to 20 inches with roots that may extend up to 24 inches underground. Unlike Shoal and Turtle grass, Manatee grass blades are cylindrical with two to four blades rising from each rhizome node. It is the second-most common seagrass in Florida waters and, as its name implies, a favorite food of the manatee.
Turtle Grass, or Thalassia testudinum, was once the most common seagrass in Tampa Bay but has been replaced by Shoal grass in many places following extensive damage by boat propellers and ongoing water quality problems. As these stresses are relieved, many of the existing Shoal grass beds could be expected to naturally change to Turtle grass beds through succession. This is happening today along the MacDill Air Force Base shoreline.
Its blades are flat and ribbon-like, growing up to 14 inches long and 1.2 inches wide from rhizomes that may be as deep as 10 inches below the surface. It typically hosts large colonies of epiphytes and its common name reflects its status as a preferred food source for sea Turtles.
