Please select your home edition
Edition
North Sails 3Di 60 - 728x90

High rainfall, lower salinity in the Chesapeake Bay

by NOAA Fisheries 15 Nov 2018 17:01 UTC
Salinity at NOAA CBIBS Annapolis May through September © NOAA Fisheries

Here in much of the Chesapeake Bay watershed, 2018 has been a bit on the damp side—to put it lightly. The area has experienced higher-than-average rainfall, leading to record amounts of water flowing off the land into the Bay.

We know what all that rain means for people—traffic-filled commutes that take longer, grumpy kids who want to play outside, rescheduled baseball games, and uncooperative vegetable gardens. But what does it mean for the Bay—and the critters who live in it?

One of the main ways that significant amounts of direct rainfall and increased freshwater flow from the Bay's tributaries can affect the Chesapeake is by lowering the salinity level of the Bay. Salinity, measured in practical salinity units (PSUs), ranges from 0 for fresh water to about 35 in the ocean. The Chesapeake Bay's geography takes it from the mouths of freshwater rivers to where the Bay meets the Atlantic Ocean, so it includes a wide range of salinities.

Salinity at a given location can vary greatly, too. Over the course of the year, salinity often follows a pattern based on rainfall in the area, so that the waters are less salty—lower PSUs—from fall through spring when the Chesapeake watershed generally experiences higher precipitation. Over the summer, salinity slowly creeps up as rainfall drops.

But some years are quite different. The following graphs show salinity at the Annapolis and Stingray Point CBIBS buoys from May through September, highlighting the average from 2011 through 2018 and a few notable years. On these graphs, the heavy black line shows the average salinity off Annapolis from 2011-18. It's interesting to track a dry year—2016—and see how salinity was higher than average, and to see what heavy rainfall in 2011 (a wet spring followed by a drier summer followed by tropical systems Irene and Lee in the fall) and 2018 (wet May, drier June, very wet July followed by Florence in September) do to salinity levels.

Approximately 40% of the freshwater flow into the Chesapeake Bay comes via the Susquehanna River. When the Susquehanna watershed, primarily in Pennsylvania and New York, gets a lot of rain, the Chesapeake Bay gets a lot of rainwater and runoff. The Conowingo Dam, about 10 miles upstream from where the Susquehanna meets the Chesapeake, provides electricity generated by hydroelectric power.

Sometimes, when the water level behind the dam gets too high, gates on the dam must be opened to increase flow in order to relieve pressure on the dam. When that happens, freshwater flow to the upper Chesapeake can increase significantly, lowering salinity. In the following graph, gate openings are indicated by increased flow from the Conowingo—leading to lower salinity at Annapolis and Stingray Point CBIBS buoys.

These changing salinity levels can affect living resources in the Bay. While how fish, crabs, and other critters may be affected by current conditions remains to be seen, scientists know from past research and observations that some living resources are affected by low salinity events.

  • Sea nettles prefer the right blend of water temperature and salinity--and with fresher water, nettle numbers are likely to be down. While this may be great news for boaters and swimmers who don't want to experience the stings they bring, sea nettles are an important part of the Bay ecosystem. They eat zooplankton, fish larvae, and also comb jellies--which in turn eat oyster larvae. So fewer sea nettles may mean fewer oyster larvae.

  • Oysters are particularly susceptible to changes in water conditions like lower salinity because they grow as part of reefs--and therefore can't move to find conditions they prefer. While lower-salinity waters can reduce the prevalence of diseases that affect oysters, they can also negatively affect oysters' ability to reproduce.

  • More mobile critters--like fish and crabs--may seek different areas of the Bay for their habitat as salinity levels change. For example, within the Chesapeake Bay, male blue crabs generally prefer fresher waters in Maryland and the upper tributaries, while females like the saltier waters in the mainstem of the Bay and Virginia. Blue crabs use different areas of the Bay--with different salinities--during their life cycle, including high-salinity areas as larvae. So different salinity levels may affect where blue crabs are found.

  • Similarly, rockfish use waters with differing salinities during their lives as well, migrating to freshwater areas to spawn, but then generally spend the rest of their lives in brackish or salt water. They grow more quickly when in water around 7 PSU then they do in water of 0.5 or 15 PSU. Depending on the timing of events, fresher water may enable more zooplankton to exist—and zooplankton is an important food for young striped bass.

  • Menhaden, which are food for many Bay species and an important commercial fishery as well, are quite salinity tolerant, and can live in fairly low-salinity waters. But how this low-salinity event may have affected how and when menhaden larvae move into the Bay is not well understood.
With so many questions remaining, scientists are eager to dig in to data. The Chesapeake Bay Program's Sustainable Fisheries Goal Implementation Team plans to focus on the topic at its December meeting, giving Team members some time to analyze data before they meet in person to discuss potential effects of this rainy year on the Bay's living resources.

Related Articles

Alaska Salmon Travels - Post 4
Join Alaska Fisheries SC in attempt to better understand hatchery-reared salmon marine survival The marking process has been surprisingly smooth, especially since most of the crew (including me) are new to the tagging game. Posted on 11 May
Celebrating 'Earth Day' every day
Celebrating Earth's ecosystem by engaging and educating communities There are a variety of opportunities to do outreach and education during this important campaign. Over the years, we've been fortunate to partner with the Waikiki Aquarium to provide our messaging to eager event goers. Posted on 9 May
Harmful algal blooms' impact on the U.S. economy
Becoming more apparent as they increase in frequency, magnitude and geographic scope The human dimensions of harmful algal blooms (HABs) are becoming more apparent as they increase in frequency, magnitude and geographic scope. Posted on 28 Apr
Scientists monitor Hawaii coral reef ecosystems
Pacific Islands research scientists embark on the first leg of a 48-day field season Coral reefs around the globe, including Hawai?i, are threatened by climate change, disease, land-based sources of pollution, and unsustainable fishing practices. Posted on 26 Apr
Lone bowhead whale sighted in Gulf of Maine
For the past few years a lone bowhead has been sighted in the waters off Cape Cod, Massachusetts This Arctic whale was more than a thousand miles from the southernmost extent of its range, socializing and feeding with other whale species before it disappeared from sight. Posted on 14 Apr
Project Protect - SCTLD Interdiction
Force Blue tackles coral disease A new video chronicles the efforts of NOAA partner Force Blue, a nonprofit that engages veterans in restoring coral ecosystems, to halt the spread of Stony Coral Tissue Loss Disease, which is ravaging the Florida Coastal Reef Tract. Posted on 13 Apr
How will changing ocean chemistry affect shellfish
Scientists at the NEFSC Milford Lab are shining some light on ocean acidification While climate change gets most of the publicity, did you know that the ocean absorbs about a quarter of that extra carbon dioxide? Posted on 12 Apr
Second young dolphin disentangled off Florida
After suffering injuries from fishing line During a routine field survey in February, biologists from the Chicago Zoological Society's Sarasota Dolphin Research Program noticed the 2-year-old male dolphin looking underweight. Posted on 4 Apr
Oyster restoration and water quality
Oysters and oyster reefs provide important benefits to the Chesapeake Bay ecosystem Oysters are filter feeders, so they help clean the water as they "eat," improving water quality. And the reefs in which they grow provide habitat for fish, crabs, and a host of other Bay species. Posted on 17 Mar
West Coast waters grow more productive
The ocean off the West Coast is shifting from several years of unusually warm conditions The ocean off the West Coast is shifting from several years of unusually warm conditions marked by the marine heat wave known as the "warm blob," toward a cooler and more productive regime that may boost salmon returns Posted on 16 Mar
Vaikobi 2019 - Footer 2Marine Resources BOTTOMClick&Boat 2019 Footer