The short version
In the late 1990s, scientists compared seven small estuaries around Waquoit Bay that receive very different amounts of nitrogen from the land around them. The more nitrogen an estuary received, the less eelgrass it had, and in the most heavily loaded ones eelgrass was gone entirely. The nitrogen didn’t poison the eelgrass directly. It fed thick mats of seaweed that blocked the light young eelgrass shoots need to get started.
What the researchers did
Waquoit Bay works like a natural experiment. Its ponds and river mouths share the same climate and tides, but the land draining into them ranges from mostly woods to densely built neighborhoods. Jennifer Hauxwell, Just Cebrián and Ivan Valiela used that range to see how eelgrass responds as nitrogen goes up.
- They studied seven estuaries in the Waquoit Bay system: Timms Pond, Sage Lot Pond, Hamblin Pond, Jehu Pond, Eel Pond, the Quashnet River and the Childs River.
- They mapped eelgrass beds in September 1997 and compared the maps with ones made in 1987.
- From November 1997 to November 1998, they tracked eelgrass shoot numbers, biomass and growth. They also measured how tall the seaweed (macroalgae) canopy grew and estimated how much light it blocked.
What they found
- Nitrogen loads varied about 75-fold. They ran from about 5 kg of nitrogen per hectare per year at Timms Pond to 407 at the Childs River.
- Eelgrass vanished above roughly 60 kg per hectare per year. Eel Pond lost its eelgrass completely during the decade, and the Childs and Quashnet Rivers, which receive the most nitrogen, no longer supported any.
- Even moderate loads did serious damage. Between 1987 and 1997, Hamblin Pond lost more than 96% of its eelgrass area and Jehu Pond lost 85%.
- Fewer shoots, not slower growth. Shoot density and biomass fell steeply as nitrogen rose. Yet the authors found the
remaining shoots generally grow at the same rate as those in low-nitrogen estuaries
(Hauxwell et al., 2003). The losses came from new shoots failing to establish or established shoots dying off. - Seaweed was the shade. The seaweed canopy grew taller as nitrogen loading increased. In Hamblin Pond, the canopy cut the light reaching young shoots by 91–100% throughout the year.
What it means for Waquoit Bay today
This study helps explain why eelgrass, which provides nursery habitat for scallops and fish, is now nearly absent from the bay. By the time of the Massachusetts Estuaries Project’s 2013 report, virtually all of the system’s eelgrass had been lost. The main exception was Sage Lot Pond, which also had one of the lowest nitrogen loads in this study.
The practical lesson is that seaweed mats and murky water are symptoms, and nitrogen from the land is the cause. The authors recommend keeping land-derived nitrogen low. That is why the towns’ cleanup targets are written as nitrogen limits, and why restoring eelgrass is one of the main measures of success. Removing seaweed or replanting eelgrass is unlikely to last while the nitrogen supply that feeds the seaweed stays high.
Next, see how much nitrogen Waquoit Bay can handle according to the state’s threshold report.
Read the paper
Hauxwell, J., Cebrián, J., & Valiela, I. (2003). Eelgrass Zostera marina loss in temperate estuaries: relationship to land-derived nitrogen loads and effect of light limitation imposed by algae. Marine Ecology Progress Series, 247, 59–73. https://doi.org/10.3354/meps247059
License: © Inter-Research 2003. The publisher makes the article free to read on its website, but it is not openly licensed and may not be republished, so we link to it rather than hosting a copy.
Drafted with AI assistance from the linked source.
