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Coastal & Estuarine Science News (CESN)

Coastal & Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries & Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bimonthly basis.

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2015 July

Contents

Blue Crabs Cross State Lines to Spawn Offshore
Unintended Consequences of Ditch Plugging
Sweeping the Beach Clean
Getting Savvy about SAV

Blue Crabs Cross State Lines to Spawn Offshore

Stock assessment approaches need to be reevaluated, authors say

Blue crabs are found in estuaries throughout the east and Gulf coasts of the U.S., where they support important commercial and recreational fisheries. Monitoring and management of this species in the South Atlantic Bight has long assumed that stocks can be assessed within individual estuaries and therefore within state boundaries. However, recent evidence suggests that many estuaries serve merely as migratory corridors and most reproduction actually takes place further offshore where state lines are irrelevant. If crabs are crossing state boundaries to spawn, assessing and managing their populations becomes more complicated.

A recent study using multiple long-term data sets found strong evidence of offshore spawning: reproductive status of females indicated a summer peak in spawning on the Continental Shelf for both Georgia specifically and the South Atlantic Bight as a whole, and sex ratios at offshore stations skewed heavily toward females (they also found a substantial decline in the number of spawners over time). Spatiotemporal analysis of the data suggested spatial grouping of spawning crabs into five biogeographic regions, which did not correspond to state boundaries. Some spawning even took place in federal waters, beyond three nautical miles from shore. The authors suggest that current stock assessment approaches, which are based on indices of spawning stock abundance on a state-by-state basis, should be reexamined to account for offshore spawning and the crabs’ ignorance of state boundaries.

Source: Ogburn, M. B. and L. C. Habegger. 2015. Reproductive status of Callinectes sapidus as an indicator of spawning habitat in the South Atlantic Bight, USA. Estuaries and Coasts (March 2015). DOI: 10.1007/s12237-015-9962-2.

Marsh pools created via ditch plugging provide poorer habitat for fish than natural pools

Since the days of colonial hay farming, New England salt marshes have been altered by humans in one way or another. We’ve come to understand that mosquito ditches, dikes, and impoundments are generally bad news for these critical coastal habitats, but recent studies suggest that one of the restoration techniques used in New England marshes may also degrade habitat quality. Ditch plugging, which is used to counteract the effects of previous ditching, is a technique in which marsh soils are used to plug the seaward end of an existing ditch to create pools that attract mosquito-eating fish and wading birds. A recent study in a Maine marsh found significant differences between marsh pools at natural and ditch plugged sites.

Mummichogs, the most common fish in these marshes and a critical trophic link, were placed in enclosures in pools at both natural and ditch plugged sites and their growth was monitored. Investigators also examined benthic communities and conducted stable isotope analysis to determine trophic structure at both types of sites. Fish growth was significantly greater in natural pools than in plugged pools (fish were 27% longer and 17% heavier). Species richness, biomass, and caloric value of benthic organisms were also higher at natural sites. Finally, stable isotope analysis revealed lower food web complexity in the plugged sites, which the authors attributed to loss of plant biomass and the physical stress of hypoxia at plugged sites. The authors suggest that the differences they observed are significant enough to warrant avoiding the use of ditch plugging in future salt marsh restoration in the region.

Source: Vincent, R. E., M. Dionne, D. M. Burdick, and E. A. Hobbie. 2015. Fish productivity and trophic transfer in created and naturally occurring salt marsh habitat. Estuaries and Coasts (May 2015). DOI: 10.1007/s12237-015-9969-8.

Sweeping the Beach Clean

Study finds no significant effects of mechanical beach cleaning at New South Wales beach sites

Sandy beaches can be messy places, littered with natural and anthropogenic detritus from algae to soda cans. On beaches that tend to be frequented by humans, beach grooming is often employed to remove trash, macroalgae, and other materials to provide a more aesthetically pleasing beach. Some studies have found that beach grooming can have negative consequences: removal of macroalgae can alter nutrient cycles, and the sieving and heavy machinery often used for grooming can cause physical damage or death to a range of organisms. But a study carried out along the central coast of New South Wales, Australia, found that sometimes beach grooming can be fairly harmless. 

The investigators examined small- (hundreds of meters) and large-scale (kilometers) effects of mechanical beach cleaning by evaluating a range of parameters at cleaned and adjacent uncleaned sites on the same beaches, as well as at different beaches where no cleaning takes place. Beaches were groomed with a tractor-towed rake 1-2 times per week in summer and 1 time every 3 weeks in winter; trash was removed and macroalgae was relocated to an adjacent spot on the beach. Little to no effect of beach grooming at these sites was found. Nutrient concentrations and abundance of bacteria, meiofauna, macrofauna, and crabs showed no clear patterns with respect to beach cleaning, and the few differences observed (sediment grain size, for example) were attributed to natural heterogeneity. 

The authors caution that these results might vary at beaches that are cleaned more frequently, that receive higher inputs of macroalgae, or where a greater proportion of the beach is cleaned. In addition, nutrient dynamics might differ at sites where macroalgae is removed entirely rather than being moved to a different part of the beach, but at least at these beaches, beach grooming seems to be ecologically insignificant.

Source: Morton, J. K., E. J. Ward, and K. C. de Berg. 2015. Potential small- and large-scale effects of mechanical beach cleaning on biological assemblages of exposed sandy beaches receiving low inputs of beach-cast macroalgae. Estuaries and Coasts (April 2015). DOI: 10-1007/s12237-015-9963-1.

Getting Savvy about SAV

What factors predict SAV abundance in Chesapeake Bay and Atlantic coastal subestuaries?

Submerged aquatic vegetation has been pummeled in Chesapeake Bay, with system-wide declines between the 1930s and the 1970s attributed to disease, invasive species, declining water quality, and hurricane damage. Thanks to aggressive management programs, SAV in some parts of the bay increased from the mid-1980s to today, but not everywhere. In fact, more bay-wide declines have been recorded in recent years. Research indicates that SAV is highly dependent on water clarity, but what other factors influence where it will thrive? A comprehensive study used univariate and multivariate approaches to determine shoreline characteristics that explain variation in SAV abundance in 100 subestuaries of the Chesapeake and Atlantic Coastal Bays systems. 

The strongest univariate predictors of SAV abundance included percent of the shoreline that consisted of forest, marsh, riprap, herbaceous wetland, and shrubland. Interestingly, the percent shoreline that was marsh was most negatively correlated with SAV coverage, followed by shorelines with large embankments and rip-rapped shoreline. The authors suggest that marsh sediments make poor habitat for SAV, which prefers sandier sediments, and marshes tend to leach materials that reduce water clarity. The multivariate analysis (regression tree approach) revealed that presence of rip-rap was most strongly associated with depressed SAV coverage. A “threshold” value of 5.4% rip-rapped shoreline was identified: estuaries with less than 5.4% rip-rapped shoreline appeared to be more resilient to change, with SAV coverage remaining more stable and increasing over time since 1984, while SAV coverage in systems above 5.4% rip-rap has not increased. 

These results indicate that managers seeking to restore or protect SAV should avoid shoreline hardening and restore hardened shorelines to their natural state whenever possible. In addition, restoration at sites near extensive marshes might not be successful even if those sites seem relatively undisturbed.

Source: Patrick, C. J., D. E. Weller, X. Li and M. Ryder. 2014. Effects of shoreline alteration and other stressors on submerged aquatic vegetation in subestuaries of Chesapeake Bay and the Mid-Atlantic Coastal Bays. Estuaries and Coasts 37: 1516-1531. DOI:  10.1007/s12237-014-9768-7.