Crabs and Mangroves in Jobos Bay Puerto Rico

Mangrove System in Jobos Bay, Puerto Rico.

One Coastal Fellow at the University of Rhode Island, Ryann Rossi, is currently working on a fascinating 3-part project with her mentor, Brita Jessen, out of both the Bay Campus at URI and Jobos Bay, Puerto Rico. Rossi’s work for Jessen’s dissertation at the Graduate School of Oceanography at URI is entitled “Ecological Effects of Nutrient Enrichment in a Coastal Mangrove System.” Jessen’s dissertation consists of three parts, one of which we discussed in detail with her and Rossi. They are studying the effects of agriculture and urban sprawl, and the associated nutrients they bring on mangrove systems in southern Puerto Rico.

Nutrients may sound good for humans, but for our ecosystems they can often mean bad news. An abundance of nutrients are introduced to ecosystems by fertilizers and pollution. They can wreak havoc upon the natural state of the environment. While the effects of nutrients on the environment have frequently been studied in developed countries such as the United States, there is a lack of studies of the problem in less developed countries such as Puerto Rico. Not only are Jessen and Rossi studying nutrient cycling in an area that has been virtually unexamined, they are studying in an area of growth and dynamism – Puerto Rico is rapidly being affected by urbanization and agricultural development. Monsanto and Pioneer – two of the largest agricultural industries in the world – have recently announced they will be expanding their facilities in Puerto Rico. Since 2003 more than 50 pharmaceutical facilities and 49 medical device companies have set up shop, and more than $4 billion has been invested in biotechnology manufacturing facilities. As the urbanized areas in Puerto Rico switch over from septic systems to sewage systems, there may be a lessening of nutrient overload from human sanitation; however, urbanization and agricultural development will likely outpace the improvements made by the transition to sewage systems.

Ryann Rossi eating lunch in the field.

The particular environment in which Jessen and Rossi are conducting their studies is the mangrove system. Mangroves are natural barriers between the sea and land and are natural carbon sinks, meaning they accumulate and store carbon dioxide (CO₂) from the sea, land and atmosphere. Like the salt marshes and wetlands here in Rhode Island, they too are feeding and nursery grounds for countless species. Tropical areas typically have lower nutrient concentrations, and thus, clearer waters than temperate regions. In order to accommodate for the lack of data, Jessen and Rossi are adapting their research from work done by Jessen’s graduate advisor, Dr. Scott Nixon. Their studies are similar to those that have been conducted in salt marshes all over the coastlines of America.

Nutrient enrichment stimulates microbial activity, which increases the rate of decomposition of materials such as leaves, seaweed and dead fish. Another key player in the decomposition of organic materials are crabs. And in Puerto Rico, there are many species. Crabs are “shredders” and the primary consumers of mangrove leaves. In the words of Rossi, crabs are “ecosystem engineers.” By this she means that they alter the habitat by increasing the rate of decomposition of mangrove leaves and other decaying material. They also dig burrows, which mix sediments and bring oxygen to roots in the peat-based sediment.In order to examine the effects of nutrient overload in the relatively unstudied ecosystem of mangroves, Jessen and Rossi have created a group of mini-ecosystem testing grounds in Jobos Bay to determine how various levels of nutrients will affect the mangrove systems. They have simulated the effects of both agriculture and urban development, while maintaining several control plots. In the urban testing areas, they are using a water-based fertilizer with a nitrogen to phosphate ratio of 16:1, while the agriculturally effected plots, use a water-based fertilizer with a nitrogen to phosphate ratio of 50:1. Both ratios are similar to what is found to contribute in temperate areas. The control plots maintain their natural nitrogen to phosphate ratios.

A quarter compared to the size of a
hole dug by a crab (ecosystem engineers).

Jessen and Rossi’s research not only tests the effects of agricultural development and urbanization on mangroves, but also – within the different test plots – what exactly is leading to the decomposition of leaves. In other words, is it microbial activity from the increased nutrients or the feeding of crabs that primarily contributes to the breakdown of leaves and increases the stability of the peat-based sediment? Jessen and Rossi know that fertilizers (nutrients) speed up microbial processes and thus, increase degradation rates with increasing levels of pollution. Sea level rise and the ability of the coastline of Puerto Rico to handle it is also an issue of concern.

In order to test the effects of anthropogenic growth, Jessen and Rossi have placed yellow leaves (those that are about to fall off the plant) inside mesh “litter” bags in each testing site so that only microbes (not crabs) can feed on them. Each bag contains approximately four grams of leaves, about 7-9 leaves. Three bags were left at each site during their last visit in early July. They’ll return to Jobos Bay in early August to weigh the bags to see how much has been broken down by microbial degradation during the interim. They will then compare the amount of microbial breakdown of the leaves within the bags to the amount of degradation of leaves that have been exposed to crabs. The crab-exposed yellow leaves have been marked by Rossi’s careful work of tying strings to each one. The final weights of the exposed and contained leaves will allow the pair to make a comparison between the amount of microbial (inorganic) decomposition and organic degradation by crabs.

Jessen and Rossi are highly enthusiastic about their research and experiences in Puerto Rico. They are very grateful to their funders, some of whom include The Nature Conservancy, the R.I. and Puerto Rico Sea Grants and the U.S. Forest Service. The URI Coastal Institute and National Estuarine Research Reserve in Jobos Bay, Puerto Rico have provided fellowships for both.

Jessen is always looking for willing and motivated students to volunteer to work with her on her exciting dissertation studies. If you have an interest, please contact her at bjessen@gso.uri.edu.

Brita Jessen of Boston, MA attended Wellesley College for her undergraduate studies and is currently studying biological oceanography under the guidance of Professor Scott Nixon. Ryann Rossi of Malta, NY will graduate from URI with a B.S. in marine biology in 2013.

Elizabeth Gooding & Lesley Lambert