Study of Lake Huron

Introduction

The Great Lakes are unique in that they are some of the largest freshwater lakes in the world by both surface area and volume. They are also newer than other large lakes. This creates a special ecosystem that is young and fragile. As such, this ecosystem has been an area of interest for many decades. Researchers have especially looked at eutrophication and the introduction of invasive species. Significant changes in the food web have been seen for both of these cases. The most recent studies have focused on the introduction of Dreissena bugensis, also known as the quagga mussel, an invasive mussel species that has both a shallow water and deep water morph (Nalepa et al., 2007).

The quagga mussels, due to its shallow and deep water habitats, have a larger effect on the ecosystem than was ever seen with the introduction of zebra mussels. In fact, zebra mussels have almost completely disappeared from the Great Lakes. Mussels filter the water, increasing the clarity, but decreasing the amounts of nutrients in the system available to other benthic groups. This increased clarity can expand the photic zone, leading to increased primary productivity. This can increase the amount of zooplankton, which can increase the amount of nutrients moving from the photic zone to the aphotic zone. However, because the mussels are effective competitors for these nutrients, this increase in nutrient cycling serves to increase the mussel population. Furthermore, because mussels spend energy in the creation of a shell, which is indigestible and therefore contributes no energy to the food web, that energy is lost from the system.

On the other hand, eutrophication, from natural or human causes, serves to increase the overall nutrient levels. This can lead to high productivity, algae blooms and “dead zones.” Dead zones are areas with high respiration, as a result of high productivity, that become oxygen deficient. Few heterotrophs can live in these zones.

Of all five Great Lakes, Lake Huron has been studied the least (Nalepa et al., 2007). Because Lake Huron and Lake Michigan have the same relative depths and are connected by the Straits of Mackinac, it has been assumed that the conditions of the two lakes are similar. Because invasive species generally start in only a few places and then spread, it is important to study their effects in all five lakes. Additionally, because there are now state and federal laws that work to regulate human influenced eutrophication, it is more important than ever to look at the lakes individually, and in greater detail. This study looks at how primary production, zooplankton, and zoobenthos vary spatially across four stations in Lake Huron. Additionally, zoobenthos is compared across time using data from a 2003 study (Nalepa et al., 2007).

Methods

Sampling was conducted at four stations in northern Lake Huron aboard the R/V Laurentian (Fig. 1, Table 1). Only stations 2 and 3 were sampled on the same day. At each station, a black/white Secchi disk was used to measure the water clarity. These numbers were mathematically converted to photic zone depth (pg. 8-9, Lab manual). Using the same mathematical conversion, photic zone depth was calculated using K-PAR determined from a Seabird CTD from each station. These were compared to the photic zone depth given by remote sensing work (Table 2). A Seabird CTD with fluorometer and transmissometer was made of the water column (surface to near bottom). These profiles were used to decide how many depths, and which depths were sampled at each station (Table 3). The purpose of collecting water samples at various depths was to determine how chlorophyll concentrations and primary productivity changed.

Chlorophyll

Water samples were taken at discrete depths using a Niskin bottle. Care was taken to keep the samples out of the light as much as possible. The samples, once moved to storage bottles, were kept cold in a refrigerator until processed. Water samples were processed in four ways- using a Phyto-PAM, using an Aqua Fluor, by doing a chlorophyll extraction, and doing a C-14 experiment. The water sampled for the C-14 experiments was collected on a different day than the initial samples (Table 1). No CTD data was recorded that day. The C-14 experiments were supervised by Gary Fahnenstiel using the “Phyto-PAM Phytoplankton Analyzer Protocol” provided at the University of Michigan Biological Station (UMBS). The chlorophyll extractions were done using the “Fluorometric Determination of Chlorophyll-a” protocol provided by Jennifer Croskrey at the UMBS. No changes in either protocol were made. Aqua Fluor and Phyto-PAM measurements were made onboard the R/V Laurentian shortly after the sample was taken using protocols provided by G. Fahnenstiel. Only the Phyto-PAM samples were dark-adapted. When possible, replicate samples were processed.

Any replicates were averaged together to reduce error. These means were then averaged over the photic zone calculated by the CTD data for each station. The C-14 experiment was used as the standard for primary production. The chlorophyll extraction was the standard for chlorophyll concentration. The various methods of analysis were compared. Additionally, comparisons were made against remote sensing work provided by the Michigan Tech Research Institute. CTD fluorescence indicating chlorophyll concentration was also used as a comparison data set.

From these comparisons, it is clear that each method has its own benefits and drawbacks. While the C-14 and chlorophyll extraction take the most time to process, they are also the most accurate. In comparison to the C-14, the Phyto-PAM was not accurate for stations 1 and 2, but more accurate for stations 3 and 4 (Fig. 2). Because the Phyto-PAM measures a short time period, any slight changes will be magnified in the data; whereas, the C-14 experiment runs over a much longer time period and minimizes that noise. Additionally, the Phyto-PAM was not working well the first day, so it is not unexpected that the readings it gave are off. Comparing the C-14 to the remote sensing, the same trend is seen, showing that the remote sensing was picking up the same relative values (Fig. 3). There is some error in the C-14 measurements for this comparison caused by having to convert the units to be comparable to the remote sensing measurements. This supports the claim that the Phyto-PAM for stations 1 and 2 were not accurate. Additionally, it is important to remember that while the C-14 is being used as the standard for comparison, it was not taken the same day that the other samples were. When possible, the C-14 data is used as the primary production rates for this study; however, where it is important to utilize the temperature structure given by the CTD, the Phyto-PAM data may also be used.

In comparing the chlorophyll extraction to the Aqua Fluor, it is obvious that the Aqua Fluor is not accurate at all (Fig. 4). The CTD readings, on the other hand, are almost identical for stations 2 and 3, but underestimate chlorophyll concentrations for stations 1 and 4. The CTD data was a quick measurement that gives good estimate data, but not concrete numbers. The chlorophyll extraction data is used as the chlorophyll concentration numbers for this study.

Zooplankton

Zooplankton was collected in duplicate at each station. The depth of the collection varied based on the depth of the site (Table 4). Zooplankton was collected using a 1.53 micron net with a 0.709 area. These samples were concentrated and preserved using formalin for later analysis. At the UMBS, the samples were diluted and at least 100 organisms were counted from each sample. Only calanoid copepods were counted and classified as small, medium, or large. The numbers of each were averaged for replicate samples. The density of organisms found and biomass are reported. Biomass was found using these classifications and standard mass for each (1.3ug small, 4.5ug medium, 14.4ug for large). Because not every zooplankton of every species was counted, this should be used as a low end estimate of zooplankton density.

Zoobenthos

Zoobenthos were collected in at least triplicate for each station. The depth of the collection is the same as the depth of the site. Zoobenthos were collected using a PONAR with an area of 0.051m². Because of this technique, it is only possible to sample small areas. This is especially important in the case of sampling for species that grow in colonies, such as mussels that tend to grow in clusters. This means that a cluster of an organism group many be picked up, creating a higher than real number. In the same breath, however, that cluster may be missed completely, creating a much lower than real number. A GoPro camera was attached for viewing bottom conditions when sampling, but was not used in this analysis because not all sites were captured using the GoPro, which could lead to inaccuracies.

The samples were washed through a 1.53micron screen, collected in sample bottles, and preserved in formalin for later analysis. At the UMBS, the samples were washed through a 1.53micron screen to remove the formalin before processing. The number of worms, midges, diporeia, spherids, mysis, and quagga mussels were counted. The density of each group was calculated. These numbers were compared to data collected in 2003 (Nalepa et al., 2007). The number of mysis is not reported since there was no comparison data from 2003.

Results

Remote Sensing

While remote sensing loses the clarity provided by individual sampling, it does provide a good general picture of conditions. The natural color map shows a large plume coming out of the St. Marys River (pg. 34, lab manual). This likely impacted site 1 the most, followed by site 2. Sites 3 and 4 seem do not obviously interact with this plume. Although this plume is seen, the suspended mineral map does not show a difference in suspended mineral concentration over the four sites (pg. 41, lab manual).

Chlorophyll

The largest amount of primary production occurred at site 1 (1.94 mgC/m³/h), followed by sites 4 (1.47 mgC/m³/h), site 3 (1.14 mgC/m³/h), and site 2 (0.56 mgC/m³/h). However, the greatest amount of chlorophyll was found at site 4 (0.74 mg/m³), followed by site 3 (0.50 mg/m³), site 2 (0.43 mg/m³), and site 1 (0.41 mg/m³). This is unexpected. Alpha values from the nearest depth to the water from the C-14 samples were also looked at. The greatest alpha value was from site 4 (0.750), followed by site 3 (0.732), site 1 (0.696), and site 2 (0.341). Therefore, although site 1 has the highest amount of productivity, site 4 has the most chlorophyll and the chlorophyll best adapted for low light conditions (Fig. 5).

Zooplankton

The largest amount of zooplankton by number per m³ was found at site 4 (2357.20 organisms/m³), followed by site 3 (1571.47 organisms/m³), site 2 (874.69 organisms/m³), and site 1 (228.86 organisms/m³). However, the greatest biomass was found at site 3 (1.06 mg/m³, followed by site 1(0.93 mg/m³), site 2 (0.73 mg/m³), and site 4 (0.59 mg/m³). While site 4 had the highest amount of organisms, it had the lowest biomass; conversely, site 1 had the lowest number of organisms, but the second highest biomass (Fig. 6).

Zoobenthos

The largest amount of zoobenthos by number per meter squared occurred at site 4 (1541.50 organisms/m²), followed by site 2 (1100.13 organisms/m²), site 1 (939.72 organisms/m²), and site 3 (247.04 organisms/m²) (Fig. 7). When looking at the breakdowns for these sites, worms dominated at sites 1, 3, and 4; however, diporeia dominated at site 2. No diporeia were found at sites 1, 3, or 4. Similarly, at sites 1 and 3, midges were the second highest organism found; but, at site 2, worms took second place, midges took third. Quagga mussels were the second highest in site 4, followed closely by midges. Quagga mussels were not found at sites 2 and 3 (Fig. 8).

The trends in zoobenthos in these four sites have changed significantly since 2003. In 2003, the greatest number of zoobenthos was found at sites 1 and 4 (3306.00 organisms/m²), followed by site 3 (2452.00 organisms/m²), and site 2 (1252.00 organisms/m²) (Fig. 7). Additionally, the overall amount of zoobenthos found significantly decreased from 2003 to 2013 (Fig. 9). On a group level, worms, diporeia, spheroids, and quagga mussels have all decreased over the last decade (Fig. 10a-e). Only the midges have a higher number at 3 out of 4 sites (Fig. 10b). Comparing the relative number of each group by site gives a clearer picture of ecosystem changes. In 2003, quagga mussels dominated at sites 1 and 4, followed closely by worms. The much lower third group was diporeia, a group no longer seen in samples from these sites in 2013. Site 3 showed relatively the same structure, but again, with quagga mussels and diporeia present. Site 2, once again, was dominated by diporeia, followed by worms (Fig. 11).

Discussion

Chlorophyll

The highest amount of primary production was seen at site 1. This makes sense because of the plume seen on the natural color map coming out of the St. Marys River (pg. 34, lab manual). The river plume can carry higher concentrations of nutrients from the river to site 1. Site 4 also seemed to be effected by this plume, which is consistent with site 4 having the second highest level of primary production. Site 1 is closer to the mouth of the river, so it gets more nutrients than site 4, accounting for the discrepancy between the two sites. Site 3 had the next highest amount of nutrients owing to its place outside of any river plumes, or artificial nutrient sources, but still being near shore. By being close to shore, nutrients from the weathering of material along the beaches and in the shallow waters can still be used. By the time those waters are moved off-shore to site 2, very little nutrients remain, leading to the lowest primary production levels. Site 2 is the deepest site and is in the middle of northern Lake Huron. This site had relatively little chlorophyll with the smallest alpha value and the least amount of production. This has significance for the zooplankton and zoobenthos populations as primary production is the base of the food web.

On the contrary, site 1 has the lowest amount of chlorophyll present. This indicates that the high levels of primary productivity are not because there is a high amount of chlorophyll. In fact, it means that the chlorophyll that are present, even at the lowest levels observed, do a much better job of incorporating carbon through photosynthesis than the chlorophyll at the other sites. This is not, however, true at low light values, because site 1 chlorophyll has a relatively low alpha value. In comparison, site 4 has the highest amount of chlorophyll and the highest alpha values, but the second highest primary productivity. This means that site 4 has a lot of chlorophyll that is good at using low light levels, which causes it to have high productivity.

Zooplankton

Site 4 has the highest number of zooplankton; however, it also has the lowest amount of biomass. Because only one species of zooplankton were counted, this could mean a number of things. Perhaps fish predation favored a smaller size. The organisms could be younger than those in the other sites; perhaps it was a group of younglings. The zooplankton population could be more diverse so that the calanoid copepods occupied a smaller percentage of the total zooplankton population at site 4. Without comparing to other species, it is impossible to know for sure. The same is true of site 1 which has the lowest number of zooplankton, but the second highest biomass. It is possible that this split could represent a split population; the young are born near site 4 and migrate to site 1 as they age. There would likely be numerous young that are lost in the move to site 1. Additionally, the older zooplankton has a much greater density.

Site 3 had the second highest number of zooplankton and the highest amount of zooplankton biomass. It is expected that the higher number translates to the higher biomass. Similarly, the lower numbers of zooplankton translate to lower numbers of biomass as seen in site 2.

Zoobenthos

Zoobenthos have decreased at every site in the past decade. The most important results seen are that diporeia have been lost from all but site 2 and that quagga mussels were not found at sites 2 or 3. The population of diporeia was the highest at site 2 in 2003. At that time, only 1 quagga mussel per cubic meter had been found. Now, no quagga mussels were found, and the diporeia population has retained over 50% of its former population. It has been previously thought that quagga mussels had the ability to take over Lake Huron, but that spread was not seen at site 2, or any site sampled in 2013. In comparison, site 4 had a middle amount of diporeia, but the highest levels of quagga mussels in 2003. Now, the number of quagga mussels has decreased dramatically, but the population of diporeia has disappeared.

Some Great Lakes researchers blame the invasive quagga mussel for changes in zoobenthos (T. Nalepa, personal communication). It is possible that the quagga mussels were the catalyst for the initial drop in zoobenthos populations. While it was predicted that quagga mussels would increase in number and take over the Great Lakes, this study found the opposite to be true. Quagga mussel numbers have decreased alongside the other zoobenthos measured. Perhaps the link draw between quagga mussels and diporeia is not as strong as some would like to think. Further research in other areas of Lake Huron should be done to see if the same trend is found throughout the lake.

Notably, the only species that has increased in number is the midges. Further study into this increase should be done. Imaginably, the increase in midge numbers could be causing a decrease in other zooplankton species. The only site that midges did not increase at was site 3. This site also had coarse sand substrate instead of the silt seen at sites 1, 2, and 4. Because zoobenthos live on the bottom, the decrease in number could be due to a less than ideal substrate.

Conclusion

This study looked at four sites in northern Lake Huron. More sites need to be looked at before any claims about the entire lake can be made. For the few sites selected, it is possible to see how location, depth, and time can change results. Chlorophyll primary production seems to be dependent mostly on nutrient levels. Zooplankton depends on location. Zoobenthos have decreased dramatically over the last decade, but how that has changed the other tropic levels is still a mystery. In fact, how the zoobenthos interact is still unknown. More time needs to be dedicated to studying this young and fragile ecosystem before it is gone.

Acknowledgements

I would like to thank Grace Lieb, Kelly Jung, and Jayna Sames for their valuable contributions to Team Superior and this paper. Additionally, thanks and appreciation goes to Dr. Gary Fahnenstiel and Dr.Thomas Nalepa for their advice and guidance throughout this study.