Seaweed And Seagrass

The Village receives many reports from residents in the summer about the increased accumulation of seaweed and seagrass along the Key Biscayne beach. Floating Sargassum seaweed (a drifting brown macroalgae) is transported from the Caribbean Sea to the Gulf of Mexico and both the Loop Current and the Gulf Stream transport the drifting sargassum through the Florida Straits to the broader Atlantic Ocean. Local currents, prevailing winds and tides move the seaweed into the Key Biscayne area where it mixes with seagrass generated locally from the ocean bottom and is deposited along the beach at high tide.

Amounts of these naturally occurring organic deposits vary by season. Residents and visitors are advised to expect large quantities during the summer/early fall. Hurricane season, from June to November, is seaweed season as well. The volume and duneward extent of the accumulations depends on the wave action/direction and is influenced by currents, the strength and direction of wind and by tidal effects. Higher winds from the NE, E and SE usually cause more seagrass, seaweed and other detritus to be deposited along the beach.

Mass accumulations of seaweed have affected Caribbean beaches since 2011 and indications are the deposits are increasingly affecting South Florida beaches. The reasons behind these excess accumulations are being investigated by marine scientists, however, increased nutrient inputs to the coastal ocean at a continental scale and warming sea surface temperatures being driven by climate change are likely contributors to the overproduction of naturally occurring sargassum seaweed.

Sargassum and rooted seagrass that naturally die and drift onto the beach are important, natural components of our marine ecosystem. Both provide food and shelter for juvenile marine life and birds.

Routine Seaweed Control Via Beach Cleaning Program

Beach Raker, the Village's beach maintenance contractor, mechanically cleans the beach within the Village limits seven days a week. Operations typically begin at approximately 6:00 a.m. and end around 10-11:00 a.m. depending on the tidal and beach conditions and how many people are on the beach. During turtle nesting season, however, the beach contractor must wait until the morning turtle nest survey has been completed before any work can commence. During turtle nesting season raking operations begin at approximately 7:30 a.m. and end around 11-11:30 a.m.  The seaweed and seagrass are typically not hauled away but are deep buried at or below the mean high water line (MHWL).  Deep burial typically involves digging a 3-4 foot deep pit that is then back filled with seaweed and is capped by sand.  Over the course of 4-6 weeks the buried seaweed decomposes and the organic matter becomes integrated into the sand at depth

At times of high winds, high tides, and high seaweed accumulations, the Village beach contractor faces challenging conditions in that there is too much seaweed/seagrass to be fully buried. All the material may not be buried as a second tidal event occurs in the afternoon after morning work is completed.  During these extreme conditions, the contractor is forced to haul the large quantities of organic material to a staging area where it is stock piled temporarily and then loaded into trucks and then transported to a landfill for disposal.  Additionally, during periods of extreme accumulation, the contractor may be asked to rake twice a day, hauling away part of the raked material and then burying the remainder.  It should be noted that hauling away seaweed is very costly and each time seaweed is raked and hauled away, there is a potential negative impact to the beach due to associated sand loss, further contributing to the problem of beach “erosion”.

Why Does The Village Deep Bury Seaweed And Seagrass?

Seaweed and seagrass are a vital part of the Village's beach ecosystem. The Village developed and implemented a beach management plan in consultation with Moffatt and Nichol (previously Coastal Systems International), the Village's long-time beach management consultant. Originally, the process of integrating the seaweed back into the sand by blading it into the top six inches of the beach at or below the MHWL (intertidal zone) was a factor in preventing beach erosion while also reducing hauling costs.  This approach has been modified, however, due to research undertaken by the University of Miami that indicated that shallow seaweed integration into the beach was promoting bacterial growth, potentially increasing the frequency of swim advisories and slowly converting the beach sand into dirt as the fraction of organic material mixed into the sand gradually increased. To address these concerns, the Village decided to deep bury seaweed to a depth of 3-4 feet and then cap with a layer of sand as is common practice along other beaches of south Florida.  This is both an essential process and a methodical one. The objective is to maintain the original profile of the beach and provide an organic matrix that stabilizes sand by burying the seaweed at the wrack line. Seaweed is not generally hauled off the beach because of the cost and the sand that is also removed with the seaweed during the hauling process. This contributes to beach erosion over time, since a component of the beach ecosystem is removed and not replaced. Also, the disposal costs of seaweed are extremely high and make this process financially impractical in the long term.

Burying seaweed along the beach helps keep several erosion "hotspots" from losing more sand. Our present beach profile shows that nearshore currents in front of Silver Sands Resort, Key Colony and Island House can be 2 to 10 times stronger than along the rest of the beach. The result is that more sand and seaweed are deposited at these locations than along the rest of the oceanfront. At the same time these currents take away, or erode, sand from the beach in front of the Silver Sands, resulting in a cove where the beach width is less than to the north or south. Erosion of sand is also more severe further south in front of the Ocean Club. The large quantities of seaweed brought onshore in the summer/early fall have been beneficial in helping "rebuild" the cove in front of the Silver Sands and in reinforcing the dunes in front of the Ocean Club. The net effect widens the sandy beach at locations where natural processes erode the beach more than in other areas, however, it is a complicated balance between leaving seaweed on the beach and not having so much seaweed on the beach that the recreational experience is compromised due to the foul smell of decomposing seaweed or huge mounds of seaweed impeding access to the water.

Upon occasion, small quantities of seaweed can be placed at the frontal area of the dunes. Typically, dried seaweed will naturally be blown up into the dunes by prevailing winds thereby supplying the dunes with nutrients absorbed by the sea oats and other dune plants that make up a healthy and stable dune system. The unique c-shape and rough texture of the grass-like sea oat fronds effectively capture windblown sand and dried seaweed and deposit it among the dune plants, further building up the amount of sand and nutrients within the system.

A healthy dune system benefits the beach and the Village in many ways. Robust vegetated dunes absorb wave energy and protect upland property in tropical storm events and are a source of sand to the beach and nearshore areas after such events. Deep burying the seaweed on site removes the accumulations before they decompose and smell and saves the Village money. Consistent removal of this material from the beach would incur hauling and disposal expenses that are not financially feasible.  The Village is always seeking innovative solutions to this challenging issue and is currently conceptualizing a pilot program on Virginia Key to test the effectiveness of composting seaweed to convert a “waste” into a resource.

Village Efforts

During “seaweed season” the Village receives an overwhelming number of inquiries regarding the state of the beach due to the huge accumulations of seaweed brought ashore by wind, waves and longshore currents.  In the Village’s continued efforts to keep our beach safe and clean for the enjoyment of residents and visitors, the Village contractor hauls hundreds of truck loads of seaweed per season in an attempt to keep the beach clear.  The number of truck hauls varies by week and by season with some seasons being heavier than other.  Truck hauling typically continues until a manageable state is reached that then allows normal operations to resume.  Presently, the Village is also working with Miami-Dade County to gain access to a staging location on Virginia Key which would make hauling seaweed off the beach a more financially sustainable management option in the long-term.  Seaweed taken to Virginia Key would be dried and then transferred to a landfill or incinerator facility elsewhere in Miami-Dade County at a fraction of its original weight.  While that is a conventional approach, a more sustainable option is being investigated whereby seaweed would be mixed with municipally generated mulch from tree trimming activity and composted and transformed into a horticulturally valuable product for use across the Village and the County.

In order to manage the Sargassum problem, the Village is always exploring new technologies and approaches to determine if they are applicable based on jurisdictional limitations, environmental regulations (federal, state, county), and cost-benefit analysis. For Example, the Village has explored the use of barriers and screens to prevent the Sargassum from coming ashore but existing environmental regulations limit the implementation of this technology, unlike in other countries with less stringent environmental protections.

While we work diligently to maintain the beach to a useability level that is agreeable to beach combers and bathers alike, stay safe, be aware of crews and equipment working the beach, and make sure to stay clear of their path as they groom and rake seaweed.



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