Monitoring the Response of the Marsh to Remedial Actions
Our laboratory carried out a 7-year monitoring program to determine how water turbidity had changed in comparison to pre-exclusion condition. We found that the effects of carp exclusion (also known as biomanipulation– meaning that the restoration relied on manipulation of a trophic level in the food-web) differed between open-water and vegetated sites. Although water turbidity was greatly reduced during the first year ofthe biomanipulation, it increased again in 1999 to a level reminiscent of pre-exclusion days at the open-water site, whereas sustained improvement was evident in the vegetated site.
Even though it is tempting to attribute the response entirely to the carp exclusion, we need to account for the effect of water level on water turbidity. In a previous study, we found that the concentration of Total Suspended Solids (TSS), which is highly correlated with water turbidity, decreased significantly as water levels in the marsh increased, and this effect must be taken into account when evaluating the effectiveness of the carp exclusion. The following graph shows that both 1998 and 1999 fell within the 95% confidence interval of the predicted line (which had been determined based on pre-exclusion data), indicating that there had not been a significant effect of carp exclusion on water turbidity once the effect of water level had been taken into account. However, a significant response was detected in 1997.
Why had there been such a dramatic drop in water turbidity in 1997, and why did the response not persist in the years that followed? The answer to this had something to do with the unsually cold spring during 1997. Recall that nowadays the marsh is normally dominated by small-bodied zooplankton that are incapable of efficient grazing. During May of 1997, however, there had been many large, non-helmeted water fleas (Daphnia) present in the marsh. The fact these water fleas had non-helmeted heads suggested the absence of planktivores (i.e. primarily alewife), which usually migrate into the marsh in May and June and are very efficient predators of the large-bodied zooplankton. In the following graph, we see that the number of alewife that migrated into the marsh in 1997 was extremely low compared with those in 1996 and 1998.
The over-abundance of Daphnia in the marsh led to unusually high grazing rates in 1997. At its peak, there had been so many grazers that the zooplankton cleared the water column 4 times daily. With this high grazing pressure, the marsh became virtually devoid of algae, and light was able to penetrate to the sediment bottom. This unusual event could not be attributed to the carp exclusion because the large Daphnia had also been noted out on the other side of the barrier, in Hamilton Harbour itself.
Usually, Daphnia do not become abundant until late summer or early fall; however, in 1997, it was very abundant during spring. This is presumably because of the delayed migration of alewife into the marsh in response to the cool spring temperatures. This delay was probably the reason that Daphnia became abundant. Once the alewife returned in mid-June, they devoured the Daphnia and the marsh returned to its turbid state.
Response of the Plant Community
Prior to the carp exclusion, there had been only a few patches of submergent vegetation in Mac Landing, a portion of the marsh that still contained residual marsh vegetation. Elsewhere, there were virtually no submergent plants.
After the exclusion, however, we could see obvious patches of submergent plants in Mac Landing, and in 1997 and 1998, patches were observed even in the open-water areas.
However, the resurgence of these plants was short-lived, as indicated by the drop in stem densities from 1997 to 2000.
Response of the Fish Community
Prior to the carp exclusion, the fish community consisted of juvenile carp, pumpkinseed, brown bullhead, with some adult pumpkinseed and brown bullhead among the many adult common carp. Following exclusoin, however, there were many more juvenile pumpkinseed as well as yellow perch, and there were also adult bluegill, brown bullhead and pumpkinseed and other sunfishes. The increased submergent community (even though it was not uniform throughout the marsh) had clearly increased biodiversity of the fish community throughout the marsh.