The Mysterious World of Buoyant Weights

As a community ecologist, I ask questions about how species interact with and impact each other. For my PhD, I have been studying the fish, crabs, shrimps, and other critters that live inside of cauliflower coral colonies. Since I spend a lot of time starring into the branches of these corals, people often think I am focused on the coral. I try to explain that I am and I am not. I love coral, don’t get me wrong, but at least currently, my focus on coral is only as a part of the associated community (granted it is a foundational species in the community which gives it extra importance, but it is also the constant as all of my

Coral colony in the lab, waiting to be buoyant weighed.
Coral colony in the lab, waiting to be buoyant weighed.

communities are associated with a coral colony). For my summer experiment, in which I was able to manipulate the community on an experimental set of coral colonies and follow how the various treatments impacted the community, I was able to also look at how the treatments impacted the coral colonies. To do this over the 2-month study window, I used buoyant weights.

Buoyant weighing was a technique I had heard about quite a bit and knew lots of people that had used it, but I hadn’t focused on myself before. It seemed simple enough, weigh the coral before and after to get a change in weight, divide by the amount of time in between weighing to estimate a growth rate. Of prime importance – weigh the coral while it is suspended in water. Ok… suspending full coral colonies in tubs of water involved some logistical sorting and ended with a carefully designed mess of lines attached to a hook on the bottom of the scale… but no real problems.

A coral is being suspended in seawater for buoyant weighing.

To take a step back, the basic idea for buoyant weights, as explained by P. Jokiel (1978), is that the dry weight of an object is equal to the weight of the object when submersed in a liquid medium plus the weight of liquid displaced by the object (Archimedes’ Principle). In the manuscript, Jokiel then walks through a few equations to get to the dry weight = the buoyant weight/ (1 – density of seawater/density of aragonite). Plugging in a density for seawater and for aragonite, it hypothetically becomes quite simple to go from buoyant weight measured to dry weight. The density of aragonite is about 2.93 g/cc. But, here comes the next challenge with buoyant weights, the density of seawater. See the density of seawater is around 1.03 g/ cc which is all one needs to know if they are wondering if it would be easier to float in the ocean or in a pool, but the density of seawater varies with temperature and salinity, so if one wanted to use it to determine fine scale differences in the weight of a coral, they need to know the temperature and salinity every time a coral is weighed… Ok, no big deal I thought, and I carefully measured the salinity and the temperature of the water every time I weighed a coral. My coral-focused friends told me to also weigh a standard – I added this to my protocol, again no real problems.

The relationship between seawater density, salinity, and temperature as presented by http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/seawater.html.
The relationship between seawater density, salinity, and temperature as presented by http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/seawater.html.

Once I was done with all the schlepping corals around, and suspending volleyball-sized and larger coral colonies in a large tub of seawater, and weighing them with a standard, and measuring salinity and temperature, I sat down to calculate seawater density… but I haven’t found that to be an easy thing to do! Since the density of seawater in a function of both temperature and salinity (and pressure though that doesn’t apply in my situation), it seems it isn’t simple to input these into an equation and get out seawater density… it was around this point that I understood the importance of the standard. The buoyant weight measurements of an object of a known, unchanging weight and size at different salinities and temperatures can be used to come up with a relationship between seawater density and temperature and salinity over the range for which measurements were made. Using this relationship, I could finally estimate seawater density for each of my buoyant weights and using this I could calculate the dry weights of the corals… phew!

After just barely surviving these processing steps, I had the initial and final dry weights for my summer manipulation. Of course the next thing was to interpret these… well it ends up the initial size of the coral can impact how much they grow… some people start with pieces that are very similar in size, so this isn’t a problem. Working with entire colonies, I had selected similarly sized corals, but they were certainly not all the same size. To work around differently sized samples some people use a wax dip to determine the surface area of the samples they are using and then standardize their growth rate with this (e.g. weight in mg/cm2 per day)… since I was working with an entire branching coral colony and I wasn’t interested in killing the corals with a wax dip, I divided the final weight by the initial weight (e.g. ln[final weight/initial weight] per day). Awesome – I had successfully calculated coral colony growth rates from buoyant weights!

Then I wanted to see how my growth rates lined up with previous studies and found that this too is not so easy to do! Studies that used a branch or piece of Pocillopora combined with the wax dip technique to estimate surface area, well their numbers were not comparable to mine since we have different units. Other studies that started with pieces that were similar in size and weight (and it seems that were able to weigh their corals at constant seawater densities) just presented change in buoyant weights! For this I could use the length of time of their study to make this into a buoyant weight growth rate and then I could select a common seawater density and backtrack my growth rates to be in buoyant weight… but as I said, comparing my growth rates with others is not that easy to do!

While waiting for the scale to settle on a weight, I am preparing more lines for hanging corals in the tub of water.
While waiting for the scale to settle on a weight, I am preparing more lines for hanging coral colonies upside down in the tub of water.

After this analysis adventure, I certainly still think buoyant weighing is a very powerful tool. As Jokiel noted –buoyant weighing can directly measure changes in aragonite, isn’t greatly impacted by things with densities close to seawater including coral mucus and commensal organisms, does not damage coral is not damaged, and can detect fine scale changes. However, it seems the research community has not come to an agreement on how to present the growth rates, or a user-friendly way to input your temperature and salinity data to get seawater density. (If anyone reads this and knows some solutions I don’t, please let me know and I’ll happily update my analysis and this post!) So, buoyant weigh away my friends – just know this technique is more mysterious than you may have hoped.

Reference:

Jokiel PL, JE Maragos, L Franzisket. 1978. Coral growth: buoyant weight technique. In: Stoddart DR, RE Johannes (eds). Coral Reefs: Research Methods. UNESCO, Paris, France. pp 529-541.