Joanne Rathe/Globe Staff

David Johnson and Kate Morkeski loaded a device that feeds fertilizer into the salt marsh on Plum Island.

Joanne Rathe/Globe Staff

For the study, researchers hauled a ton of fertilizer to the salt marsh every four days or so in the spring and summer.

Joanne Rathe/Globe Staff

David Johnson checked the tank where water is mixed with fertilizer and released during the rising tide.

Joanne Rathe/Globe Staff

David Johnson offloaded fertilizer from a boat onto a station in the marsh.