Warming Waters on Cape Cod: An Exploration of Eelgrass Patterns with Marine Biologist Holly K. Plaisted


First Seagrass of the Season!

APRIL 25, 2022

Following Holly at First Encounter Beach, Eastham, MA April 25, 2022

On this bright and sunny morning, I excitedly drove to First Encounter Beach in Eastham on the Cape to meet with local biologist Holly Plaisted. I had previously met with Holly on Zoom at Zostera meetings and heard all about her interesting work on warming waters and its effect on seagrass. Holly is a marine biologist who lives and works on Cape Cod, and was mentored by Fred Short.

Hey Holly, wow low tide is amazing this time of year, the sand goes on for miles! I wonder why it called First Encounter Beach?

This is where the pilgrims first encountered Native Americans, before Plymouth

Fascinating, you mentioned you never knew much about the beach here even though you were working on the Cape

Yes, I never spent any time on the Bay side beaches since I was always working in Wellfleet. So I stayed up there, but since I moved to about 8 minutes distance from here, I come every morning to this beach with the dogs. I spend a lot of time just looking for eelgrass out here.

Oh yeah, eelgrass..what do you do with it out here, actually what is your job?

I am a biologist, my specialty is water quality and seagrass monitoring. So through that work I get pulled into a lot of opportunities . So we can apply our long term monitoring to research questions. Answer them, like, Ok we see these patterns but why?

I know that you are also a former student of Fred Short, and Fred is like the guru who..How would you describe Fred

He is Passionate about Seagrass… He’s been a very kind patient teacher to me over the years. Doing science, writing papers and giving presentations does not come easy for me. But Fred has always been positive and encouraging to me– still is!

I only met with Fred or two hours in 2016 for my first blog interview, it was transformative! He is a great mentor and has shared so many great resources and people with me. I have never looked at seagrass the same! So back to seagrass, what happening here on First Encounter Beach?

Well, I take pics and document the gps of them to monitor them..

We walked towards the small patch in the vast stretch of sandy beach

I tried to eyeball the location first couple of times, but it was way easier when pinned on the phone.



What about the seagrass here on First Encounter Beach?

Well, I take pics and document the gps of them to monitor them..

We walked towards the small patch in the vast stretch of sandy beach

I tried to eyeball the location first couple of times, but it was way easier when pinned on the phone.


How often do you come look at it?

I first found it in January, and since then I come check on it a couple times a month and last week. I looked at it and found Ruppia. Do you know Ruppia?

Not really, is it a seaweed?

It’s actually a different type seagrass. It doesn’t get as much attention as eelgrass- our regular seagrass, its a different species, It is more ephemeral and doesn’t provide the same ecosystem services as Eelgrass. However it still provides some of the services since it baffles wave energy, and stabilizes sediment.

These are small, since they don’t need to be so tall.


How does it get sun energy with so much volume of water going in and out every

6 hours?

Yes, It goes up to here ( Holly lifts her hands above her head to about 6 Ft) When the water goes up and down every 6 hours the light gets reduced through the water column. Out here it’s also very turbid when we have a little wind..this all effects the photosynthesis.

So how do they manage to stay alive with all these adverse conditions?

These are survivors…Feel how warm it gets Its over 30 degrees..when the waters low its going to get..hot..but it gets sun..Its a tiny patch here..but they are able survive.

Look how cute these young eelgrass looks..

They might not be young, it might be the morphology that makes them survive here..short and dense..see how they have off shoots here, that is indication that they are more established

Photosynthesis enough to keep up the carbon balance.


How can you tell the difference between just seedlings and the ones with  rhizomes here?

I did see seedlings here, they will be singular plants kinda on their own, and don’t have a linear pattern like the rhizome this time of year (early in the sping). Like this one, it’s on its own.( she points to an individual plant) If we pulled this up It probably has the tiniest root.  Where as this one (Holly points to a linear arrangement of seagrass )is on a long rhizome.


What do the rhizomes tell you?


You know how they have these segments on the rhizome If segments are tight together, really tight segments between the nodes. They are growing slow.


It tells us how quickly or slowly a plant is growing.


If the internode is really long, like they have 3 inches between the next node, they are growing really quick.



So its good to have them spaced out between nodes, so that means they are more prolific are growing really quick?

Yes the longer the internode the more quickly they are growing This is clonal – asexual reproduction.


Also, is that called Asexual reproduction? When they produce more offshoots?

Tell us what a rhizome is and all this sexuality how does it work with seagrass?

The rhizomes are what store carbohydrates, the expansion of the rhizome node, thats how the plant expands in space. That is Asexual reproduction.


Where as with seeds, it is sexual reproduction. So they just flower and drop the seeds or depending on the currents, they can be swept away a foot and settle elsewhere. Where as if the whole shoot breaks off, those can float real far and then distribute seeds in father away locations. They can float and distribute seeds which is sexual reproduction.



So is this group of seagrass from seeds?

Yes, they had to start from seeds, since it can’t start from nothing, there are no other rhizomes near by. Could have been successful seed recruitment so I will keep looking this small patch this year to see if they produce flowering shoots this year. Usually seedlings, which I am pretty sure these are not seedlings, since they are on rhizomes..Usually seedlings will not produce seedlings the same year. Just like your perennials in the garden, some years they don’t produce any flowers, they just have vegetation. Eelgrass are the same way. But there are some forms of eelgrass that are true annuals and complete their whole life cycle in one season.


Ok if seeds are sexual reproduction how does asexual reproduction work?

I am just learning about sexual reproduction. This year I am making an effort to keep track of the shoots, record info on how- the different timing of their shoots. 


When you look , how do you record information?

I  collect preliminary info to spur maybe a research project. I

1.     Wanna know where I am

2.     Water depth is

3.     When know if I see reproductive shoots or not?

4.     If they are reproductive shoots – what phase are the flowers at?




So how do you know which ones are reproductive soots?

They are branched A little taller than the vegetive shoots and have a yellowish color.,


[I think we’re looking a ruppia here] Look how tiny the rhizomes, barely under the sediment surface. So the reproductive shoots of Rupia will get ten times taller than their vegetative shoots



What is this ? ( I point to a chunky green slimy vegetation)


Codium is an algae, it’s an Asian invasive species.

We’ve noticed a huge increase presence of Cape cod bay. I spoke with fishermen who does clam dreading in the bay. He said that Codium can get so thick on the bottom that their gear bounces of off it.


Fascinating. They look very hearty..Does it have any good purposes..?

I don’t know.

It will attached to any shell, so it helps with holding down sediment.

If Codium gets more pervasive what happens to the seagrass? Can it cover the seagrass?


Yes it’s a competitor and we can get declines in seagrass,

part of our work we doing we’ve  noticed a drastic decline in seagrass in Wellfleet. So we have drafted a research proposal to figure out what is leading this loss we are seeing there.


Is this in Wellfleet?

Yes, in Wellfleet, over 5 square milesof all of eelgrass was lost-  accounts for almost half  the loss in all of the mapped Massachusets. In one town, Billings Gate Sholes..



I did read in a nature book, about  how the Cape has these big bouts of sand drifts..how the sand moves in big chunks, have you seen these? Could that cover eelgrass?


Yes, we see this, eelgrass patches get buried in storms. We can go day to day and see patches disappeared the next day.


There is no way to remove the sand or shift it elsewhere?

No, we cant control the sand movement happening. But that’s natural these movements have been happening forever!. That’s natural these eelgrass’ adapted to these disturbance events. Being eroded, being buried, but what needs to be there is a seed bank for recovery.


Oh so we make a seed bank so we can go get new seeds to plant?

No, I talking about the natural seed bank.

The plants are producing seeds. So think of it like a forest. A forest goes up in flames, how does it come back?


The baby trees grow?





So you would go tap into the seed bank?

Well, thats possible. That would be a way for mitigate the loss in dire areas. However, I am talking about the natural seed bank. That just stays naturally in an environment that had a healthy productive eelgrass meadow. So if in July all these huge meadow, produce millions of seeds. Those seeds stay in the sediment. If there is a huge heatwave, and that heat wave kills say 50% of the eelgrass, the above ground material. The eelgrass cant stand without above ground material. The seeds provide the potential for that meadow to recover through seedling recruitment. Seeds start to germinate, and that is the future of the meadow. But if you don’t have the eelgrass meadow to make the seeds, you don’t have the seed bank.


What Are seed banks?

Seed banks can be two things. First the institutions where they store seeds for food and agricultural crops – warehouses or whatever. And there is also be natural seed banks in the sediment (or soil if we’re talking about terrestrial environments)


The [natural] seed banks could possibly explain why we go through periods of no seagrass, and then surprise, some start growing right?


Yes, it could explain the unexpected spurts



How does oxidization work in nature, I went to Virginia ( where they have  the largest eelgrass restoration site with a huge man- made seed bank) and they would oxidize everything manually.

Yes they have a great man made seed bank


How does nature provide oxidation?

Seeds at low oxygen and cold temperature might actually be triggered to germinate. So what I believe they are doing in Virgina is to prevent them from germinating so they can plant them later in the year.


Yes, that sounds right, they hold on to the seedlings so they can wait out the super hot weather. Talking about heat waves, at the last eelgrass presentation, you had mention studying temperature on eelgrass here on the Cape. Can you expand on that?


Since we started monitoring seagrass out here, using the seagrass net system. We did temperature loggers at the transects. And they sit there year round and measure water temperature at 30 minute intervals.


What do you use that data for?


We use the data for a lot of different things. That recent paper, we used the temperature data and the seagrass net data that monitors the North East to see if there were relationships between seagrass absence and abundance to summer water temperature, with the hypothesis that increasing thermal stress in the summer time like with heat waves  would have a negative impact on the eelgrass cover.


What did you find out?

When water temperatures 1 year prior to the seagrass monitoring were higher than the sites average temperatures eelgrass was less likely to be present.  

So we looked at the site for over 15 years


Then we averaged the site temperature- then we compared the one years temperature.


If that one years temperature was over the average site temp, the following year, the probability of eelgrass was reduced.


Is it the next following year that eelgrass gets impacted?

That’s with the data we had, thats what showed. Up. Ideally

What is interesting is that impact of high temperatures persisted into the next year.

Its the effects of these warming effects have longevity. Its not one heatwave and we are done


What is happening on the Cape with temperatures?

Well, they are getting warmer and we are having more  marine heatwaves in the winter- 5 consecutive hot days means a marine heatwave, it doesn’t have to be in the summer.


Ha, March must of been a marine heatwave then?


Yeah, probably..


This report that you compiled, does it show that we are trending towards more heat waves? Dare I say, should we be expecting more warming temps?




Whats your hope with this research?


My hope is that Its not just about temperature. What the high temperature does is that it exacerbates the other stressors that we have some control over. Like nutrification, nutrient pollution. We have control over these stressors.


My hope is that the research allows management decisions that reduce or eliminate those stressors that impede eelgrass growth. Or recovery in areas it once grew. For example, in mooring fields, these are things we have control over in Pleasant Bay, Wellfleet, all over the Cape.


Are mooring fields those donut rings in the eelgrass that boats make?


Thats right, they have a chain that attaches to the the mooring that attaches to the boats, they  swirl and circle around scours the bottom of the eelgrass beds, leaving these donut shapes. It also reduces water clarity by resuspending those sediments into the ater column that effects the seagrass’ ability to photosynthesize.



Thats something I thing we can control like Gloucester has those new types of moorings that are less detrimental to eelgrass?


Yes, they are called conservation moorings.


Do we have any control over the over-nutrification problem? Can you briefly explain what it is and how we can make change?


We can improved water water treatment, reduce or elimate fertilizer and pesticise use in coastal watershed.


Your research is super important for all of us. The fisherman, clam diggers, those of us who like to swim on the Cape like me- we are really benefactors of this silent plant that gives us so much life- Here are some examples:

 It is pretty amazing the wealth of biodiversity that exists on our shore lines! Is there any other thoughts you’d like to share?



Also I hope the research inspires us to try to increase the strength of our existing meadow like they did in Virginia with seeding, so maybe we need to try out different methods of restoration.


Maybe meadows where they are on the tipping point, where the bed is not doing great, maybe we can seed..like overseeing our lawn, lets over seed our eelgrass beds.



We need to have a larger  infrastructure to help do the conservation work like in Virginia.



Big holding tanks, and to house the reproductive shoots, harvesting them..they are great conversations..We need to talk about these things..we’ve had smaller groups working out these topics that Tay leads, you’ve been to some right?



Are there any things that Cape Coders could do to help in eelgrass conservation today on the Cape?



I think citizens can get in touch with their select board and conservation commission and ask what the towns are doing to protect and restore eelgrass within their town. Every town on the Cape has lost eelgrass over the past 25 years. Citizen should know what their towns are doing about it.





What a magnificent way to spend a morning on the beach with you Holly, I learned so much it might take me a few years to process the information. Thank you for the wonderful research you are doing and for sharing your knowledge with me!