Katie Gavenus: There’s More Than One Way to Catch a Copepod, May 2, 2019

NOAA Teacher at Sea
Katie Gavenus
Aboard R/V Tiglax
April 26 – May 9, 2019

Mission: Northern Gulf of Alaska Long-Term Ecological Research project
Geographic Area of Cruise: Northern Gulf of Alaska – currently on the ‘Seward Line’
Date: May 2, 2019

Weather Data from the Bridge
Time: 2053
Latitude: 58o 53.2964’ N
Longitude: 148o 34.4176’ W
Wind: 10 knots, West
Seas: 4-5 feet (Beaufort Scale 5)
Air Temperature: 7oC (44o F)
Air pressure: 1016 millibars
Overcast, no precipitation

Science and Technology Log

We were able to deploy the bongo net at 3 more stations on the Middleton Line before rough seas compelled us to head to some of the more sheltered sampling stations in Prince William Sound. (Sidenote: we did see a handful of myctophids in the last two hauls we did on the Middleton Line. Those are the lantern fish I was keeping a special eye out for after learning that they can be important black-legged kittiwake food this time of year.)

Though it complicates the schedule for the rest of the cruise, spending last night and today in Prince William Sound turned out to be fortuitously timed for this blog about zooplankton.

Along the Middleton Line, the night zooplankton crew deployed the bongo net, which does a cumulative sample from the surface through the water column to a specified depth and back up to the surface again. In general, the depth that we are deploying the bongo net to is 5-10 meters above the ocean floor but in deeper water we stop at approximately 200 meters. My understanding is that the bongo net is a good and straightforward way to get an overall look at zooplankton abundance and community structure.

In Prince William Sound , we deployed a Multinet instead, which has several nets with only one of them open at any given time. When the Multi reaches a specific depth range (like 200-150 meters), a computer signals the first net to open and it is towed until it reaches the next target depth after enough water has passed through it. That net is then closed, and a second net is opened at the next shallower depth. So on and so forth, until the Multinet has collected a sample at each of 5 discrete depth layers in the water column. Both the collection of samples and processing of them on deck take longer than the bongo nets. However, the major advantage of the Multi is that researchers can get a better sense of what is happening at different depths in the water column, rather than lumping zooplankton over 200 meters of depth all together like the bongo does.

Ability to analyze zooplankton abundance and community structure at different depths is important for a number of reasons. In a nearshore ecosystem like Prince William Sound, there are often significant gradients of salinity, temperature, nutrients, dissolved oxygen, and trace minerals as well as primary productivity. Data from the CTD and water sampling at various depths at each location can be compared to where certain species or life stages of zooplankton were found using the Multi, and this can help the LTER project to better understand what conditions support different types of zooplankton.

Another reason that a Multinet can be a useful tool relates specifically to the life history of common copepods in Prince William Sound and the Gulf of Alaska. Common in both coastal and offshore waters in this region, three species of the copepod Neocalanus gorge on the spring bloom of phytoplankton in order to build up lipid stores. These copepods go through different life stages. During the day, a different set of nets (called a CalVET) with finer mesh are deployed to 100 m and brought up vertically through the water column to catch zooplankton. Copepods from the CalVET sample are sorted by species and life stage to better understand inter-annual variability in their seasonal cycle and distribution.

At the Prince William Sound station, almost all of the Neocalanus observed were in the N. flemingeri copepodite-5 stage, which is the stage just before they reach adulthood. In the next month, the C5 females in particular will store as much lipid as they can. In June, perhaps even late May, the N. flemingeri will molt into adults and swim down in the water column to approximately 400 meters or greater in depth. Here the female adult copepods will diapause, a hibernation-like process through which their metabolic activity slows significantly as they ‘overwinter’. They spend July through February or March in deep water. They do not feed in this adult stage, so as C5s the females must accumulate enough lipids to last through 7-9 months of diapause and the production of eggs! They tend to lay eggs beginning in December through January or February, and die soon after they release the eggs. The males on the other hand die in June shortly after mating and do not diapause.

An aspect of the LTER research related to copepods analyzes how successful different Neocalanus spp. are when it comes to finding enough food to build up lipid stores. One approach to answering this question involves photographing Neocalanus spp. with a specialized microscope and measuring the length and width of their lipid ‘bubble’ relative to their body size. This visual assessment is really cool, because you can actually get a solid sense of it here on the boat. Another approach utilizes analysis of gene expression called transcriptomics to ascertain if the copepods are food-stressed. Different markers will indicate whether the copepods are building or burning lipids. The copepods for this analysis are collected on the cruise, but must be processed in a lab on land, so it can be a while before the data is ready.

Whether or not C5 Neocalanus spp. are able to sufficiently fatten up is a crucial question. If they can’t store enough lipids in April, May, and June, the adult females will not make it through diapause to reproduce. If this is true for a large portion of the females, it can dramatically impact copepod abundance the following year. And of course, these future changes in copepod abundance could impact carbon cycling and will likely ripple through the food web. Even more immediately, many species of vertebrates rely on lipid-rich C5 Neocalanus spp. each spring. If the C5s are starving, birds and fish that depend on these fatty snacks may not be able to feed enough for their own reproductive success.

Although the abundance of Neocalanus spp. caught in the CalVET was lower than expected for Prince William Sound, the ones that were caught generally displayed robust lipid stores. Out along the Middleton Line, the copepods had smaller lipid stores and most of them were a life stage earlier in development. Generally, Prince William Sound has an earlier phytoplankton bloom than the more offshore areas, so it isn’t surprising that the Middleton Line copepods aren’t as fat yet. As we sample at the Seward and Kodiak Lines, I will be peering over shoulders at the microscope to get a glimpse at the oh-so-important bubbles of lipid in the copepods.

Consider now that you’ve read multiple paragraphs about the unique natural history of just one subset of zooplankton – Neocalanus flemingeri and other species in the genus Neocalanus! These organisms are a crucial part of the flow of energy, carbon, and nutrients through the ecosystem. But they are just one part of a much more diverse zooplankton community. Alongside Neocalanus spp. we’ve seen a plethora of other copepods, euphausiids (krill), decapods (usually larval shrimp), and ostracods, as well as sea jellies, ctenophores (comb jellies), chaetognaths (arrow worms), and larval fish. And that’s not even discussing microzooplankton like ciliates! As a community, and as individual species, these zooplankton are important players in the Northern Gulf of Alaska. I am constantly impressed by the depth of knowledge the LTER zooplankton researchers have about these organisms, and simultaneously astounded by how much is still a mystery. The world of zooplankton is fascinating, and so many wonderful questions remain!

Personal Log

I think I’ve finally shifted over to a more nocturnal schedule. I slept most of the day, but once again managed to wake up just in time to see some whales as I drank my ‘morning’ tea. There were a couple of minke whales, which is cool in and of itself, but the magnificence of the minkes was eclipsed by the sighting of a sperm whale. Sperm whales are somewhat common out in the Gulf, where they dive thousands of feet in search of squid. However, it is very unusual to see them in the shallower waters of Prince William Sound.

After dinner, the Tiglax veared in to Icy Bay for some additional CTD and CalVET samples. This was a special treat, as it allowed for a spectacular view of Chenega Glacier as well as the harbor seals and birds that hang out amongst the chunks of ice in the bay. We had another chance to go out in the zodiac skiff and were able to slowly make our way through some of the smaller icebergs for a closer look at the glacier. It was an incredible evening!

Did You Know?

Zooplankton utilize many different strategies to find food. Many species of copepods feed primarily on phytoplankton. Some of these herbivores utilize chemoreceptors to ‘smell’ the phytoplankton while others rely more heavily on mechanical receptors positioned along their antennae to listen for their food. Other copepods are predatory, with sharp claws for grabbing their prey. Many other species of zooplankton are predatory too; they attack, entangle, or paralyze other zooplankton to consume. But the options aren’t limited to herbivore or carnivore! Last night, as we were checking out one of the zooplankton samples, we found a copepod with a parasitic isopod; this isopod sucks nourishment from the copepod as an intermediate source before moving on to its final host, a glass shrimp. Though I didn’t see one in person, I was also told about a parasitic copepod that lives in the gills of cod.

Question of the Day:

Does oyster farming reduce local plankton biomass to a degree that is visible in adult populations of organisms like steamer clams?

Question from Kim McNett, artist & science educator, Homer, Alaska.

Though no one aboard specializes in oyster diets, I shared this question at dinner and the plankton experts were willing to make some conjectures. Clam trochophore larvae are fairly soft-bodied, so it is likely that oysters could consume them. A first step to answering this question would be to find out what size range of plankton oysters consume and compare that to the size of clam trochophores for the species of interest. If the clam trochophores are significantly larger (or smaller, but that is unlikely) than the size-fraction targeted by oysters, there probably isn’t much predation going on. But if the trochophores line up with the size eaten by oysters, then predation is definitely possible. Another step would be to figure out if clam larvae overlap in space and time with hungry oysters.

We also discussed whether or not oysters might compete with clams for food, and adversely affect the clams in this way. Generally, the consensus was that there might be some impact immediately around oyster lanterns but that over larger scales the impact would be negligible. Because oysters are farmed in lanterns suspended in the water column, and clams are located in the benthos and intertidal areas, there may be some niche partitioning. That is to say – the oysters are likely feeding on different plankton than what would reach the clams. To answer this question more fully, once could look at what size-fraction of plankton oysters feed on and compare it to the size-fraction consumed by clams. One could also look at the movement of water to try to determine whether the same plankton that drifts through oyster lanterns is likely to also drift into the intertidal and benthic locations where clams are located.