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A Tale of Plankton and Sewage

Welcome back for another update! Our urban kelp survey is now 100% live. Thanks to you all we BLEW through our first round of images (seriously, over 8000 images in exactly one month!!!) , and we are now ready to start classifying kelp!

Today we’ll briefly talk about our choice for our California site, Los Angeles. 

Image Description: Satellite image of Los Angeles
Image Source: Google Earth, accessed 6/12/2020

LA is currently the largest in our study, at least in terms of population/density. It saw its biggest population boom in the early to mid 1900s as both the entertainment and war industries grew. Unfortunately for us, we don’t have satellite data from this time. While we are missing the major population growth spike, Los Angeles continues to be a major metropolitan hub – it is the 2nd most populated city in the United States, and one of the largest worldwide!

The kelp forests in California have a patchy (ha ha…) history that is often closely linked to human activities – If you ask a phycologist about it, you’ll almost certainly hear about the Point Loma and Palos Verdes kelp forests. These two coastal kelp beds are located near San Diego and Los Angeles, respectively, and are infamous for sustaining massive kelp loss in the first half of the 1900s. This was due to a variety of factors, some natural and some human, but it seems that urban sewage discharge was the straw that broke the kelp forests back.

In the case of Point Loma, a broken sewage pipe discharged almost 200 million gallons of sewage into the kelp forest, leading to quick and major kelp loss. Luckily the area quickly recovered after the pipe was repaired – one advantage of kelp’s fast and furious life style is that it can quickly repopulate a large area if environmental conditions are restored! Palos Verdes has had a longer road to recovery, although recent restoration efforts have been successful in recovering many acres of kelp forest.

Discharge from sewer systems is bad for kelp in several ways.  The discharge tends to be full of sediments (solid material suspended in the water). This can prevent growing kelp from getting sufficient light or can even completely bury small individuals.  Another negative impact associated with these outfalls is nutrient pollution. In some cases, chemicals like ammonia can spike to toxic levels, causing immediate damage.

 In other cases, elevated nutrient levels can simply “fertilize” the water. Your first reaction might be to think that this would be good for the kelp, and maybe in a vacuum it would be, but nature isn’t that simple. In this case, the complication comes from phytoplankton, which can compete with kelp for light by forming floating algal mats that block light and shade the seafloor.

Kelp grows quickly, but not as quickly as phytoplankton. When water nutrient levels are elevated (a condition called eutrophication), plankton growth can ramp up into what is called a plankton bloom. That’s a lot of words to say that these algal blooms can have major effects on the ecosystem. This picture might “clear” things up (ecological puns never get old…).  These thick green algal blooms are more characteristic of fresh water, but I think this does a good job of illustrating how severe these can be.

Image description: Body of water that is overgrown with a thick algal mat. A researcher is collecting a water sample
Image source: Tim Otten, Oregon State University

The immediate, kelp-relevant effect is a reduction in light at the sea floor.  The longer term effects can include reduced oxygen levels in the water, creating uninhabitable “dead zones. This low-oxygen condition is called hypoxia and is a result of rotting phytoplankton. The typical life cycle of a bloom goes something like this:

  1. Eutrophication event (discharge or runoff causes elevated nutrient levels in the water)
  2. Algal bloom (opportunistic plankton grow exponentially given high nutrient levels)
  3. Overgrowth (algal population exceeds available nutrients; growth eventually stops as nutrients are depleted) – Light levels beneath the water’s surface reduced
  4. Decomposition (algal bloom dies and decomposes. Bacteria associated with decomposition consume oxygen, creating a dead zone) – Hypoxic conditions form, many animals die or leave

Kelps are potentially affected by almost every step of this process. They are shaded out by the initial bloom and then potentially buried in detritus as the plankton decomposes. Many of the animals found in kelp forests are vulnerable to the dead zones caused by algal blooms. Loss of these kelp forest residents can further destabilize the ecosystem.

While the stories of Point Loma and Palos Verdes unfortunately played out mostly outside of Floating Forest’s time window, they serve as clear reminders that human activities can seriously affect kelp forests. Things like sewage discharge and eutrophication are global issues; California may be a high profile example, but is far from the only one. As we continue to explore the effects of urbanization, we will be sure to include as many local factors (such as sewage treatment outfall points) as possible to best understand each of our sites.

Kelp and the Big City

If you’ve read through our “about section” or seen our blog in the past, you’ll know that kelp is threatened by climate change. This is perhaps the main reason why we are so interested in kelp in the first place, and so far Floating Forests has allowed us to build some of the most complete kelp forest datasets around. Soon we will be adding a new set of images to the project, this time examining a few key, high-risk locations with a fine-toothed comb. We’re interested in how coastal construction or development might impact kelp forests – it’s time to take our kelp research to the city! We will be taking a close look at kelp forests in California, Chile, Argentina, Australia, and New Zealand.

While global climate change is extremely important, there is great value in examining smaller scales as well.  In fact, studies such as this one have shown that kelp forests are strongly affected by local conditions! As you might expect, changes to these local environmental conditions can have significant effects on the local wildlife.

We call changes that affect the environment “drivers”. For example, a construction project could “drive” environmental change by releasing sand and dirt (which are collectively referred to as “sediments”) into the water. When two different drivers interact and affect each other, we refer to them as “synergistic”.  Occasionally they can create especially favorable conditions, but often one exacerbates the negative effects of the other, so we call these “synergistic stressors”.

One way to think about this is to say that one driver can amplify the effects of another. For example: a marine ecosystem might be able to handle a 2° Celsius increase in average temperature without collapsing. Separately, it also might be able to handle the effects of a large construction project. However, the effects of both at the same time could prove catastrophic: organisms already stressed by warming might not be able to cope with disturbances caused by the construction, and most importantly, things might get bad much more quickly than we might expect.

Because an emoji is worth a thousand words:

This example is one of the many that play out across an ever-developing world. One consistent trend over the last several decades has been a large increase in coastal population. To say this more plainly – more people than ever before live near the coast. As coastal cities grow, constant expansion is needed in order to keep up with the population. This conversion of land from natural to urban terrain is known as “urbanization” and brings with it a host of environmental impacts – some of which are potentially harmful for kelp forests.

By digging into the satellite record, we can get an idea of how kelp forests in urban areas have changed over the last few decades in response to this trend of development. What’s really exciting about this opportunity is that with the power of citizen science we can cover a lot of ground: we want to compare cities across the globe in order to build a better understanding of how kelp in different places is affected by human activities. We expect there to be many interesting differences between these locations – it would probably be more surprising to learn that kelp in Australia was acting exactly like kelp in Chile!

Stay tuned as we prep for the launch of this exciting phase of Floating Forests – details about our study sites and more about the connections between urbanization and kelp coming soon!

Evaluating mutualism between fish and large brown macroalgae

We just finished a three year project in which we developed a series of mensurative and manipulative experiments on different sites along the Central-Northern Coast of Chile. Study sites on the project where selected under different parameters such as open access areas (meaning that anyone can go and get fish and seafood) and protected areas called areas of management and exploitation of benthic resources (AMEBRs) where there are fishermen who care for benthic resources and only they can catch them. To measure the state of kelp forest and the relationship between the brown seaweed and fish, 4 scientific divers with various tasks recorded and monitored kelp forests of these sites twice a year for three years.

a) kelp in good condition in MEABRs and b) in open access for fishing sites. Photos A. Pérez-Matus

a) kelp in good condition in MEABRs and b) in open access for fishing sites. Photos A. Pérez-Matus

The results obtained by these divers were very interesting, the AMERBs sites have more adult and juvenile brown macroalgae per unit of area, and adult plants are larger, because they have more foliage. At these sites there is also a greater number and biomass of fish, all in comparison with the forest of brown macroalgae of free access sites for fisheries. Free access sites had a higher number of grazers like sea urchins and snails and small herbivores that eat brown macroalgae. To evaluate these results in field, experiments where set up to estimate the growth of the blades of brown macroalgae Lessonia trabeculata (see figure 1a) in the presence and absence of herbivorous snails measuring over months. The seaweed in the presence of this snail not only stopped growing, but also declined (see figure 1b). The next step was to experiment in the laboratory the feeding behavior of the herbivores in the presence and absence of potential predators (fish) for this aquarium with herbivores and macroalgal tissues were used. The experiment showed that snails fed less in the presence of fish. The mere presence of fish caused snails to climb to the top of the aquarium and spend more time there. Then it was confirmed that fish generate indirect positive effects on the brown macro algae.

Other positive direct effects of fish on brown macroalgae were also evaluated. In the laboratory, it was confirmed that in the presence of fish and filter feeders such as mussels, macroalgae are kept in better condition. They do not lose their blades (leaves) and grow faster than seaweed that only had the presence of filter feeders. The nutrients that fish generate as particulate organic material are dissolved by the filtering procedure of mussel then are used by macroalgae. It turned out that the seaweed benefits from the nutrients provided by the fish and accelerate their growth.

Figure showing the experiments herbivory on reproductive tissue of the kelp Lessonia tabeculata by the herbivore “jerguilla”, Aplodactylus punctatus: a) experimental mesocosms; b) delivered algae as food; c) indicates consumption and bites by adults jerguillas. Pictures from Catalina Ruz.

Figure showing the experiments herbivory on reproductive tissue of the kelp Lessonia tabeculata by the herbivore “jerguilla”, Aplodactylus punctatus: a) experimental mesocosms; b) delivered algae as food; c) indicates consumption and bites by adults jerguillas. Photos from Catalina Ruz.

The brown macroalgae generate different services to the fish species, providing habitat for early stages of fish and food indirectly (many fish feed on small crustaceans such as amphipods, gastropods, isopods inhabiting kelp) and directly, as their own tissues are food for herbivorous fish. Herbivorous fish can consume algal reproductive tissues. In these tissues spores live; spores are the seeds of macroalgae. We found that these reproductive tissues that were consumed by fish herbivores may reduce the epiphytic (seaweed on tissues) and chemical load that are produced in these tissues (to deters small herbivores). The seeds of the algae can be released more quickly once passes through the digestive tract of fish and may supply more nutrients for growth. We also found that the seeds are viable and germinated with the same speed as those of reproductive tissue control. Herbivorous fish can be vehicles that disperse seeds macroalgae as these spores have very little mobility (see the photos of the experiments).

This research project may generate different conclusions. One is that as the brown macroalgae are important habitat and food for fish species. The fish in turn provide benefits to macroalgae. The importance of fish is notorious and is expressed at different spatial scales (1 m to km) where the presence of fish and other agents can modify the landscape. We can conclude that in the absence of fish, macroalgae lose their condition (assessed as foliage and growth), which could bring significant consequences for artisanal and recreational fishing. Also, there are no management plans in Chile led to the fish. Considering the importance of these coastal environments, consequences will be negative to kelp ecosystems if limitations for catching reef fish in Chile do not apply.

Floating Meadows?

While Floating Forests is focused on canopy forming kelps – typically Giant Kelp, Bull Kelp, Sea Bamboo, and others these are not the only kelp. In New England, for example, kelps are only a few meters tall, and create vast meadows instead of ‘forests’. Take a look at this amazing video by Brian Skerry from Cashes Ledge featuring some great comments from our collaborator, Jon Witman.


Endangered Kelp Forests in Tasmania

Not seeing much kelp in the Tasmania images taken in recent years? This may be due to the fact that this region has seen dramatic declines in Giant Kelp (Macrocystis) over the past few decades. This decline has been linked to warming sea temperatures off the east coast of Tasmania. The loss of this critical habitat has been so dramatic that the Australian government has listed the forests as endangered. This is the first time that an entire ecological community has been given this kind of protection.

One major goal of this Zooniverse project is to better document these declines. While SCUBA diving is a great way to see and study kelp forests, divers can’t get everywhere and so there are many places where we don’t know how much kelp has been lost. With your help we can observe the entire coastline of Tasmania! And we will get many views of this coastline each year going back to 1984! So don’t get discouraged if you aren’t seeing kelp in the more recent years. These zeroes are incredibly important data for us. If we start seeing kelp in those same places when we look at images from the 1980s and 1990s then we can measure how much kelp has been lost.

Read more about the Tasmanian kelp declines here and here.


A nutrient starved kelp frond. Photo courtesy of the SBC LTER