The past 18-20 months have been sad ones for me as a kelp forest ecologist. I feel like a coral reef ecologist who has been marooned down in Antarctica for a long time. With very little of our island kelp forests surviving, and those that have sickly due to nutrient starvation, there is not much for me to study in my home waters. Of course I take episodes like this as a chance to study the dynamics of kelp forests, their demise and (hopefully) recovery. However, every time I submerge and see the devastation, I shed tears. Fortunately, just like the ocean, they are salt water… but they do make my eyes burn. In my upcoming columns, I’m going to describe what has been happening in our waters during this long warm water period, and hypothesize about what is happening and what may follow.
Back in the late 1990s when I was Vice President of the Catalina Conservancy, we held a number of public meetings to let the community know what we were doing in the areas of ecological restoration, science and education. The most contentious of these meetings focused on our efforts to remove the non-native mammals like pigs and goats that were causing serious impacts to the native plants of Catalina. I frequently heard people say “Why are the plants so important and not the animals?” Sadly, this showed a tremendous lack of understanding about natural ecosystems. Of course these non-native mammals were also affecting the lives of animals… our native insects, birds, reptiles and mammals.
In our island’s marine environment we have been seeing a devastating decline in the native algae or seaweeds. Technically these are not “plants,” but as some of the primary photosynthetic organisms in our ecosystems, they fill the same role as the plants do on land. Organisms that photosynthesize are critical for capturing sunlight and converting it into usable forms of energy (food) suitable for many fish and invertebrates to chow down on. Without this conversion, there would be little energy available to fuel the species comprising the local ecosystem. Herbivores would have no “plant” material to feed on and therefore predators would have no prey to munch. Without “plants,” the production of oxygen would also be limited and the fish, invertebrates and, yes, even human beings would have little to breathe.
So “plants” and algae are critical components of almost every ecosystem with the exception of those in the deep ocean (but even there they are important). If the algae cannot survive, neither can the food webs and ecosystems based on them. What would cause them to disappear? On land, droughts like the one we are currently struggling with can cause high mortality in plants. At least many of them drop seeds that survive in a dormant state until rains return. Of course drought is not an issue in the marine environment! There a lack of nutrients is often the limiting factor. Here on Catalina we have few gardens and almost no lawns, but if you grew up somewhere else, you probably realize that fertilizer and/or mulch is often necessary for plant growth. Even those with nothing but potted house plants realize this. Unfortunately, I have a brown thumb so nothing grows in my house… not even cactus!
There is a well known relationship between nutrients and water temperature that I’ve never fully understood. It states that at about 68° Fahrenheit, nutrients begin to plummet. Our unusually warm water thanks to The Blob offshore is therefore low in nutrients. When nutrients are low for just a period of a week or two, much less many months like the current situation, algae begins to become nutrient starved and growth may stop. Diseases may take an additional toll. The weakened giant kelp (Macrocystis pyrifera) also succumbed to the strong surge of Hurricane Marie a year ago August which tore much of it out. For many months the little kelp remaining in the dive park has not been able to reach the surface due to poor growth. Much of it is deteriorating due to lack of nutrients and succumbing to disease as well… and the forecast el Niño hasn’t even really hit us yet!
Other native kelps in shallow waters such as feather boa (Egregia laevigata) and southern sea palm (Eisenia arborea) have also suffered from lack of nutrients. Much of the feather boa has disappeared in the shallows. What is left of the sea palm kelp often has blades (“leaves”) that have deteriorated or even dropped off. Without the blades to capture sunlight, very little photosynthesis can occur. The alga cannot grow and herbivores like kelp snails cannot use it for food. The male sheephead that chow down on the snails have to seek alternate food.
Usually deeper water is colder and nutrients may be more plentiful at depth even when they are low in near surface waters. The giant kelp that is persisting, albeit weakly, is coming from deeper depths. There are also species of deep water kelps like oarweed (Laminaria farlowii) and fringed sieve kelp (Agarum fimbriatum). Generally they exist in an environment where nutrients are more plentiful, but over the last 12 months I’ve recorded temperatures in the mid to upper 70s F at depths of 100 feet. These deep water kelps are also suffering from nutrient starvation and many of them disappeared during the warmest months.
So, many of our native algal species no longer dominate the rocky reefs of the dive park and many other regions of the island’s leeward coast. Our local invertebrates that depend on them like abalone, kelp snails, kelp crabs, sea hares, etc., etc. cannot find food. Add to this decline the dominance of the non-native alga from Asia, Sargassum horneri (aka “devil weed”). It does better in low nutrient environments than our native species and has been dominating the rocky reefs for the past few months. In cool years the canopy of the giant kelp forest can shade it out, limiting sunlight from reaching it so it doesn’t dominate as exclusively. There is no canopy this year.
During warm water episodes, there is nothing to shade the Sargassum and little competition from our native seaweeds. Once this “devil weed” dominates the reefs, it prevents our native algae from coming back by outcompeting them for space, light and nutrients. There is some speculation that the “devil weed” may also produce chemicals that inhibit the growth of other algae, including our native species, in its vicinity. Most of our resident herbivores do not find it tasty because it produces bitter chemicals known as polyphenols to which they have not adapted. Therefore the fish and invertebrates that feed on algae find little palatable to munch on. Kind of like when I go to my refrigerator late at night (or early in the morning).
There is an entirely different energy pathway in our kelp forests and surrounding waters that has also been impacted. All my readers know of the plankton that drifts in the seas around us, and that the base of food chains in that community is the “plant” plankton or phytoplankton. Just like their much larger cousins the kelps, phytoplankton are affected by low nutrient levels. When they become nutrient limited, they no longer grow and reproduce to create a continual food source for the animal plankton (zooplankton) that feed on them, or the baitfish, blacksmith and other fish species as well as the filter feeders that munch on plankton.
So we have gone from a nutrient rich ecosystem of two years ago with a wide diversity of native algae, to a nutrient poor environment with a single seaweed dominating most of the reef. This is a serious problem for the stability of any ecosystem. Diverse ecosystems with a wide variety of different species are almost always more stable than a simplified ecosystem dominated by just a few species as we see today. In the next two columns I will look at how this drastic change in the primary producer (photosynthetic) level affects all the other levels of our kelp forest ecosystems. Yes, plants and other photosynthetic organisms are critical in ensuring a supply of “energy” in all ecosystems. Without them, we humans would be in deep doodoo.
© 2015 Dr. Bill Bushing. For the entire archived set of over 650 “Dive Dry” columns, visit my website http://www.starthrower.org
Image caption: Giant kelp with deteriorating blades and stipe with no blades; southern sea palm healthy and with blades all gone.