Pond Characteristics

Drivers of Change

Activities on the land and characteristics of a watershed may have potentially negative impacts on a pond’s water quality and ecosystem health. These activities and characteristics that have a negative impact are referred to as stressors. For example, a cranberry bog adjacent to a pond could be a stressor, as nutrients from fertilizers used to grow cranberries could be released into the pond. Similarly, golf courses may produce runoff carrying fertilizers and pesticides that travel to nearby ponds over the ground or through the groundwater. 

Commission staff identified drivers of change using GIS data and our understanding of ponds. We then looked to see if individual drivers of change impacted pond water quality. We were unable to draw any correlations between a single driver and pond water quality, so we then looked at all of the drivers of change cumulatively on a single pond.

To do this we assigned scores to the drivers or stressors. The drivers that have a detrimental impact on a pond were given a score of one, and drivers that do not have a detrimental impact on ponds or may even help a pond to cope with a stressor, such as a centralized sewer system, were given a score of zero. Other more complex drivers, such as the amount of impervious cover, were assigned a score on a scale. The scores were summed up, resulting in a cumulative stressor score for each pond, ranging from 2 (minimal stress) to 20 (highly stressed).

Using this score ponds were binned into categories ranging from minimally stressed to highly stressed. More highly stressed ponds should be considered high priorities for monitoring and restoration activities, while less stressed ponds should be considered for protection strategies. Restoration activities could focus on managing identified drivers and preventing any new stressors. See the interactive map tab which shows all Cape Cod ponds ranked by their cumulative stressor score.

Interactive Map

This interactive map shows all of the ponds on Cape Cod ranked by their cumulative stressor score.

From a regional perspective, a possible way to prioritize ponds for management action based on their cumulative stressor score could include the following tiered approach. Towns may have additional criteria to help prioritize ponds for action, as well as funding and logistical considerations.

Minimally stressed (1-5): Develop a management plan to protect the ponds.
Some stress (6-10): Continue monitoring to ensure the pond does not further degrade while also developing a plan to protect the pond.
Stressed (11-15): Determine possible remediation projects that can be quickly implemented, such as stormwater management practices, and assess priorities for future management. Continue monitoring.
Highly stressed (16-20): Prioritize for mitigation. Identify and implement major rehabilitation strategies to reduce or remove stressors. Continue monitoring.

Phosphorus Hotspots

To help identify ponds where immediate, meaningful stormwater management action may be taken, phosphorus loading hotspots on a pond shore were identified. These hotspots were generated from hydrologic response units (HRUs), which are 100-square meter areas of common physical characteristics that are expected to respond to precipitation in a similar way. HRUs in pond buffers were run through a hotspot analysis tool that identified locations within each 100-foot buffer likely to have high phosphorus levels.  

HRUs were additionally used to generate several stressors: flow rate, the load of total suspended solids, total phosphorus, and total nitrogen. These stressors were included in each pond's cumulative stressor score, and for this study were also mapped. Refer to the Interactive Map on the previous tab to see where hotspots along pond shorelines and impervious surfaces (e.g., roads, boat ramps) intersect. These locations will need to be confirmed as hydrologic response units do not take into consideration existing stormwater management practices.

For additional information about HRUs check out the Commission's Stormwater Impacts Tool.  

Takeaways

There are many factors involved in a pond’s healthy ecosystem, and the GIS analysis conducted as part of the Freshwater Initiative considered a limited set based on available data. The ponds with the highest scores aren’t necessarily the ponds with the highest trophic status. There are likely stressors that have not been quantified in the cumulative stressor score, as well as other variables that help a pond cope with stressors, such as short residence time and volume of a pond. Some of these variables that help a pond manage stressors were not included because the information is not available for all ponds; inclusion for some ponds and not others would skew the scores.

This study and the regional pond analysis have helped us understand that more data are needed, and more research needs to be done, including filling critical data gaps:

• Numerous ponds don’t have recorded maximum or average depths
• Pond volumes should be calculated and watersheds delineated so volume-to-watershed ratios can be determined for all ponds monitored
• Groundwater quality, specifically the concentration of total phosphorus is needed.

Regardless of pond health, there are easy-to-implement strategies that would benefit a pond. The HRU tool used in this project located phosphorus loading hotspots on pond shores. Towns could reference this map to implement stormwater control measures.

For a deeper dive into this study, please see the Pond Characteristics Analysis Summary.