Lower Food Web

Discover the foundation of the aquatic food web

A photo of a dragonfly larvae.

Everything is connected through the lands, waters, and all living things. We have roles and responsibilities to look after the lands and waters. The lands and waters sustain us and provide us with everything we need. Indigenous knowledge systems and languages are connected to the land.

Our Relationship with the Land and Water, Anishinabek Nation

Every fish in Georgian Bay feeds on tiny aquatic plants and animals at some point in its life. This includes phytoplankton, zooplankton, and benthic invertebrates.

Phytoplankton, just like plants on land, require nutrients, such as phosphorus and nitrogen, and sunlight to grow. Zooplankton get their energy from eating phytoplankton or other zooplankton. Finally, benthic invertebrates can eat phytoplankton, zooplankton, or other invertebrates.

A photo of phytoplankton in a microscope slide.

This collection of life is known as the lower food web. It feeds larger aquatic species, including small baitfish, large predator fish, young turtles, loons, bald eagles, and many others. Without a healthy lower food web, the higher levels of the food web simply cannot thrive.

Studying the lower food web can provide information about overall aquatic health in a number of ways:

  • Phytoplankton levels are related to nutrients that support growth, such as phosphorus, and may drop as nutrient levels decrease.
  • Zooplankton levels can indicate changes in phytoplankton and other dynamics, such as contamination.
  • Benthic invertebrates have different tolerances to changes in water quality. A reduction in pollution-sensitive invertebrates can indicate deteriorating water quality.
Zooplankton, Cladoceran, Daphnia mendotae with eggs. July 2010.

Phytoplankton Trends

From the late 1980s through the mid-1990s, phytoplankton in the open waters of Lake Huron underwent very little change. There were 40 common species, and all major groups were similarly abundant over this time period.

Historically, nutrient inputs into the lake from tributaries in the spring would contribute to a surge in phytoplankton growth, referred to as the spring phytoplankton bloom. This was the time of year with the greatest amount of phytoplankton available in the water column. Between 2003 and 2008, scientists first noticed a significant decrease in the spring phytoplankton bloom. Since then, this major episode of primary production has remained almost entirely absent in offshore waters.

Conditions that support phytoplankton changing and are not well understood. Possible factors contributing to shifts in abundance and community composition are:

  • Low nutrient levels in offshore areas
  • Invasion of quagga mussels in deep waters
  • Changing water temperatures due to climate change.

 Did You Know?

The Severn Sound Environmental Association monitors 14 open water locations throughout Severn Sound. All Severn Sound bays have shown a decrease in the total biovolume of algae since 1973. Zooplankton diversity has fluctuated, but total density of crustacean zooplankton has been declining.

Zooplankton Trends

Between 1998 and 2006, zooplankton populations in Lake Huron declined dramatically, due in large part to a 95% decline in the abundance of herbivorous crustaceans such as cladocerans. Zooplankton populations have not rebounded in more recent years, and unfortunately the zooplankton groups that experienced the largest declines (such as cladocerans) were the ones that fish consumed the most.

Researchers have not yet determined the exact reasons for the decline of zooplankton, but there are a number of factors that are thought to have played a role, including:

  • Changes in nutrient availability and the related loss of phytoplankton
  • Changes in the fish community
  • Introduction of invasive species, including the predatory spiny water flea and zebra and quagga mussels.

Climate change also poses a potential threat to zooplankton. Researchers have observed increasing water temperatures and decreasing ice cover in all of the Great Lakes. Warmer water may alter temperature-dependent growth and reproductive rates, potentially favouring species that can adapt to new, warmer temperatures. In addition, earlier spring warming could lead to earlier spring phytoplankton blooms, potentially creating a situation where peak food supply for herbivorous zooplankton occurs too early for them to take advantage of it. Without adequate food available at the right time, zooplankton populations could decline, ultimately affecting higher levels of the food web.

The relationships between warming temperatures and zooplankton dynamics are not well studied in the Great Lakes and represent an important area of future research.

A graphic displaying the food chain of aquatic life.

Benthic Invertebrate Trends

The benthic invertebrate community in Lake Huron and Georgian Bay has undergone dramatic changes in the past several decades, with the introduction of invasive zebra and quagga mussels and the almost complete loss of Diporeia, a shrimp-like freshwater crustacean.

Surveys in recent years have shown that in the offshore waters of Georgian Bay, 71% of total benthic density (the number of benthic organisms in a certain area) is made up of oligochaetes (aquatic worms), followed by quagga mussels and Chironomidae (non-biting midges and bloodworms) making up a combined 13%. Looking at biomass (total weight of organisms in a given area), quagga mussels accounted for 98% of total wet biomass in Georgian Bay!

A photo of a Amphipod Diporeia spp.

However, in the nearshore areas of eastern Georgian Bay (with water depths less than 20 metres), the abundance of zebra and quagga mussels was found to be relatively low, especially when compared to similar habitat in Lakes Erie and Ontario, but with a wide distribution.

In the past, Diporeia were the most abundant benthic organism in the cold, offshore regions of Lake Huron and Georgian Bay. Diporeia live on the lake bottom, in the upper few centimetres of sediment, and feed on settled plankton from the water column. Their bodies contain 30 to 40% fats and oils, making them a vital energy source for fish and crucial to the entire food web.

Between 2002 and 2017, Diporeia populations experienced serious declines. In 2002, mean Diporeia densities across all depth intervals ranged from 1,400 to 1,700 individuals per square metre. Fifteen years later, in 2017, the range in mean densities was two to five individuals per square metre at the shallowest and deepest depth intervals. This represents the loss of a major food source for many Lake Huron fish species. As a result, fish populations have been forced to change their diet and move to areas with more food.

A graphic illustrating the decline of Diporeia in Lake Huron from 2000 to 2017.

The crash of Diporeia happened around the same time as the number of zebra and quagga mussels was rapidly increasing, but the connection is not yet well understood. Possibly, the mussels are using nutrients in the nearshore areas and on the lake bottom, reducing nutrient availability for Diporeia, zooplankton, and fish that live offshore. Further research is needed to understand the causes behind the Diporeia decline and the implications for the entire food web.

A photo of Quagga mussels at a depth of 300 feet in Georgian Bay.

Zebra and Quagga Mussels

Shelled organisms on the lakebed, such as freshwater clams, are part of the native Great Lakes ecosystem. Unfortunately, two non-native species of mussels have invaded Georgian Bay waters over the past three decades. Zebra mussels entered our awareness as a safety concern for swimmers due to their sharp shells. Water shoes became the norm. Zebra mussels also started attaching themselves to water intakes, causing restrictions and blockages and resulting in millions of dollars’ worth of clean-up maintenance to keep the pipes flowing. Those mussels started to disappear, and many people thought the problem had disappeared with them. But that’s when a worse issue arose.

Quagga mussels followed, moving from their native eastern European waters into the Great Lakes, and they began to outcompete the zebra mussels for the algae and nutrients both species needed to grow. The calcium-rich Georgian Bay offshore waters now contain trillions of bottom-dwelling quagga mussels. These mussels have, in turn, removed the food source of a major shrimp-like prey species called Diporeia, which themselves are a fatty and protein-rich food source for Great Lakes fish species. The result has been a collapse of Diporeia populations, as well as fish populations, in the offshore waters of the Bay.

These quagga mussels are particular about the algae they consume, spitting out toxic blue-green algae in favour of other algae species. Algae blooms are increasingly likely to be toxic, as over half a million residents of Toledo, Ohio experienced in 2014, when a do-not-touch-or-use order was issued for the water coming out of their faucets. Boiling water won’t break down those toxins, so one needs to be extremely careful in order to stay safe.

Quagga mussels have also been implicated in the periodic bird and fish die-offs seen along Georgian Bay shorelines. Scientists suspect that quagga mussels are ingesting the botulinum toxin that causes botulism disease in birds and fish. The toxin is believed to bioaccumulate in the mussels, the invasive round goby fish eats the mussels, then other fish and diving birds consume the round gobies and die as a result of botulism poisoning.

Quagga mussels grow right on top of native species of shellfish and disrupt their basic functions, choking them out, starving them, and ultimately killing them. Further research is needed to better understand the potential effects of quagga mussels on benthic communities in Lake Huron, particularly in offshore waters.

An Uncertain Future

Nutrients, including phosphorus, are in major decline in offshore waters. Phytoplankton and zooplankton levels are also dropping. Diporeia, once the most abundant benthic food source in Lake Huron, has been decimated.

What effects will this have on species higher in the food web? Are invasive species, combined with climate change and warmer waters, creating a biological desert on the bottom of Lake Huron and Georgian Bay? More research can help to answer questions like these.

GBB works with townships in the region to monitor benthic macroinvertebrate communities in area lakes over time.

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