DOI: 10.25338/b8c065
title: Recovery of planktonic invertebrate communities in restored and created tidal marshes along the northern Gulf of Mexico

description: A significant amount of tidal marsh restoration has occurred over the past
 two decades. However, restored marshes often fail to recover biological
 structure and ecosystem functions comparable to reference marshes. We
 implemented a 13-site inventory to evaluate the recovery of zooplankton
 and meroplankton abundance and community composition along the Mississippi
 and Alabama Gulf coasts. Understanding the recovery of zooplankton and
 meroplankton communities in restored marshes is critical, as many
 planktonic invertebrate species contribute to nutrient cycling and food
 web dynamics. We found that zooplankton and meroplankton communities in
 restored tidal marshes were comparable in total abundance, taxonomic
 richness, and taxonomic composition to communities observed in reference
 tidal marshes — with composition being driven mainly by surface water
 salinity. But zooplankton and meroplankton communities in restored marshes
 did have lower evenness and diversity than comparable reference marshes.
 These results suggest that zooplankton and meroplankton communities in
 restored marshes along the Mississippi and Alabama Gulf coast tend to
 recover after 7–34 years and support robust populations of prey items for
 larger, ecologically and economically-important species (e.g.,
 fishes). 
description: Site Descriptions and
 Marsh Restoration We used a series of 13 tidal
 marshes in coastal wetlands along the Alabama and Mississippi, USA coast
 to evaluate the diversity and composition of planktonic invertebrate
 communities in restored tidal marshes of the northern Gulf of Mexico
 (GOM). Ten of these tidal marshes had brackish surface waters ranging in
 salinity from 2–13 ppt. The additional three marshes had fresh surface
 waters with a salinity of 0.1 ppt. All tidal marsh sites were exposed to
 meteorologically influenced, diurnal micro tides with a maximum amplitude
 of 0.8 m. Our 13 tidal marshes were
 mainly nested within three watersheds along the Mississippi and Alabama
 coastlines. Three marshes, one natural (Fowl River Natural) and two
 restored (Fowl River CON-1 and CON-2), were clustered along the West Fowl
 River (30°21'59.5"N, 88°09'33.2"W) on the western side
 of Mobile Bay. One natural (Grand Bay Natural) and one restored (Grand Bay
 Restored) marsh were located approximately 0.5 km from the mouth of Bayou
 Heron inside the Grand Bay National Estuarine Research Reserve
 (30°24'43.6"N, 88°24'19.1"W). Finally, one natural
 (Weeks Bay Natural) and two restored marshes (Weeks Bay Restored and
 Magnolia Springs) were in Weeks Bay, located on the eastern side of Mobile
 Bay (30°23'49.2"N 87°49'42.3"W).
 Five restored marshes were
 located outside of these three main waterways: Perdido Beach, Helen Wood
 Park, Deer Island 1, Deer Island 2, and Greenwood Island. Perdido Beach is
 a small brackish marsh along Perdido Bay, east of Mobile Bay
 (30°20'28.5"N, 87°29'56.6"W). Helen Wood Park is a
 brackish marsh along the western coast of Mobile Bay near the mouth of Dog
 River (30°34'11.5"N, 88°05'06.9"W). Deer Island 1,
 Deer Island 2, and Greenwood Island are open-coast marshes located west of
 Mobile Bay along the Mississippi coastline (Deer Island 1 and 2:
 30°22'17.8"N, 88°50'07.5"W; Greenwood Island:
 30°20'00.1"N, 88°31'00.7"W). 
 The ten restored tidal
 marshes included in our study were developed using three distinct
 strategies. The two restored tidal marshes along the West Fowl River (Fowl
 River CON-1 and CON-2) were created in the 1980’s as mitigation efforts
 for a coal and grain facility by harvesting pine savanna habitat,
 excavating topsoil down to the water table to allow for hydrological
 connectivity with the West Fowl River, and planting the area with native
 marsh plants. Five of the restored tidal marshes were living shorelines
 (Grand Bay Restored, Helen Wood Park, Weeks Bay Restored, Magnolia
 Springs, Perdido Beach) that range in age from 7–19 years old and are
 green infrastructure planted with native marsh plants. Finally, three of
 our restored tidal marshes were created 7–19 years ago by the Mississippi
 Department of Marine Resources through the beneficial use of dredge
 materials (Deer Island 1, Deer Island 2, and Greenwood Island).
 Specifically, these beneficial use marshes were built by spreading dredge
 material to establish a marsh platform and allowing vegetation to
 naturally populate the site. Most (n = 9) of our tidal
 marshes had plant communities dominated by native rushes in the
 genus <em>Juncus</em>. Eight of these marshes were composed
 mainly of <em>J. roemerianus </em>(a common species in
 oligohaline and brackish marshes of the northern GOM), and one was
 composed of <em>J. effusus </em>(a common species in
 oligohaline marshes of the GOM). The remaining four marshes (Helen Wood
 Park, Deer Island 1, Deer Island 2, and Greenwood Island) had plant
 communities dominated by <em>Spartina alterniflora </em>and
 <em>S. patens</em>, grasses common in brackish and saline
 marshes along the Atlantic and Gulf coasts. In our study, tidal marshes
 dominated by <em>Juncus </em>species were mainly oligohaline,
 while marshes dominated by <em>Spartina </em>species are
 mainly brackish. At all tidal marshes, dominant plant communities covered
 30-95% of the marsh surface.  Invertebrate
 Surveys At all 13 tidal marshes, we
 surveyed subtidal invertebrate communities using a combination of passive
 collectors and vertical plankton tows. We used passive collectors to
 target benthically-recruiting meroplankton communities (e.g., swimming
 crabs). Vertical plankton tows were used to target the total zooplankton
 community, which are commonly monitored in restored tidal marshes to
 understand the availability of prey resources for higher trophic
 levels.  Both survey techniques were deployed in ~0.5 m of water at every
 marsh between July and August 2021. We sampled in the late summer since
 this is when the abundance of juvenile fishes, known to consume
 zooplankton communities in tidal marshes, peaks throughout Mobile
 Bay.  <em>Passive
 Collectors</em>:<em> </em>We
 deployed three cylindrical passive hog’s hair collectors at each tidal
 marsh site. Specifically, the collectors consisted of a 25 cm x 20 cm
 piece of hog’s hair (Rheem Dust & Pollen High Performance Indoor
 Air Filter) zip tied around a 76 cm long PVC pipe with a 3.8 cm diameter.
 Passive hog’s hair collectors have frequently and successfully been used
 to monitor meroplankton communities in tidal habitats. We deployed the
 passive collectors in the shallow subtidal adjacent to the tidal marsh
 platform by inserting the sampler’s PVC into the soil until only the
 portion covered by the hog’s hair was above the soil. This standardized
 the portion of the water column that interacted with our passive samplers
 across tidal marsh sites— the bottom 25 cm of the water column. We chose
 this orientation to maximize the colonization of our passive collectors by
 both intertidal and subtidal meroplankton. While our sample size of three
 collectors per marsh is small, past studies using passive hog’s hair
 collectors have suggested that three replicates is sufficient to
 characterize recruiting meroplankton communities. 
 Passive collectors were
 deployed in July 2021 and were collected two weeks later in August 2021.
 Upon collection, we placed each individual passive collector in a 1L glass
 mason jar filled with 70% ethanol. Collectors remained preserved in 70%
 ethanol until they were processed by rinsing the hog’s hair filter with
 70% ethanol for 5 minutes, with the rinse ethanol and storage ethanol from
 the mason jar collected in a 33 cm x 23 cm glass pan with a 1 cm x 1 cm
 grid on the bottom. We let the ethanol in the glass pan settle for 5
 minutes before we scanned the pan for any macroinvertebrates (e.g.,
 snails, juvenile crabs, shrimp). When macroinvertebrates were found, they
 were identified to the lowest taxonomic group possible using a standard
 key, counted, and removed from the sample at this time. We then surveyed
 the remaining ethanol for microinvertebrates by randomly sampling at least
 three, 1 cm<sup>2</sup> grids in the glass pan for
 identification. We sampled more than three grid samples if the third
 subsample included two or more taxa that had not previously been observed
 on the collector. All microinvertebrates were identified under a
 dissecting microscope to the lowest taxonomic group possible using a
 standard key. After identification, proportions of each taxonomic group in
 the subsamples were extrapolated to total density of each taxon per
 m<sup>2</sup>. <em>Vertical
 Tows</em>: To compare the meroplankton
 composition on the passive collectors to that available in the
 zooplankton, we conducted three vertical tows at each site in August 2021.
 Each vertical tow was conducted in habitat adjacent to the passive
 collectors [depth = 0.3-0.8 m; volume = 2.4 ± 0.2 L (mean ± 1SE)] using a
 plankton net with a 30 cm diameter opening and 80 µm Nitex Nylon mesh. Tow
 volume differed slightly between marshes because marshes varied in their
 local hydrology. However, zooplankton abundance and diversity were
 unaffected by tow volume. Samples were preserved in 70% ethanol until
 processed in the laboratory. We chose to use vertical tows, rather than
 horizontal tows, because we wanted to sample the same section of the water
 column that was in contact with our passive collectors to facilitate
 comparisons of these distinct datasets. We processed all vertical tows
 by emptying the full sample into a 33 cm x 23 cm glass pan with a 1 cm x 1
 cm grid on the bottom. We let the sample in the glass pan settle for 5
 minutes before we randomly sampled at least three, 1 cm<sup>
 2</sup> grids in the glass pan for species composition and density.
 We sampled more than three grid samples if the third subsample included
 two or more taxa that had not previously been observed in the sample. We
 identified all organisms to the lowest taxonomic group possible using a
 dissecting microscope and a standard key for the northern GOM. After
 identification, proportions of each taxonomic group in the subsamples were
 extrapolated to total density of each taxon per m<sup>3</sup>.
 Data
 Processing <em>Recovery
 trajectories:</em> We calculated the total
 abundance, species richness, species evenness, and Shannon Index of
 zooplankton and
 meroplankton communities observed on each passive collector
 (n = 3) and in each vertical tow (n = 3) collected at every tidal marsh
 site. Species richness was calculated using the Margalef index, species
 evenness was calculated using Pielou’s evenness index, and Shannon Index
 was calculated using a log base ‘e’. We had to exclude one of the 10
 restored sites (Helen Wood Park) from our passive collector dataset
 because two of the collectors deployed at this site were damaged.
 To evaluate the recovery of
 invertebrate communities in tidal marshes along the northern GOM, we
 calculated the recovery trajectories of planktonic invertebrate total
 abundance, species richness, species evenness, and Shannon Index. Prior to
 the analysis, we paired all restored tidal marshes with the most
 comparable reference tidal marsh sampled. Pairings were based on the
 relative surface water salinity. We chose to group reference and restored
 tidal marshes based on surface water salinity because prior studies
 indicated that salinity influences invertebrate community structure. We
 obtained response ratios for all diversity indices at each restored tidal
 marsh site using the equation: ln((Xrest+1)/(Xref+1).
 <em>Xrest</em> is the mean value observed in the restored marsh and <em>Xref</em> is the mean value observed in the reference marsh. The value “1” was added to the numerator and denominator to avoid zeros. Positive response ratios thus indicate that restored marsh zooplankton and meroplankton communities outperform reference marsh communities, while negative response ratios indicate that restored marsh zooplankton and meroplankton communities lag reference marsh communities. We chose to calculate the response ratio, rather than directly compare diversity indices between restored and reference tidal marshes, because salinity can affect the diversity of marsh planktonic invertebrate communities — making direct comparisons between fresh and brackish tidal marshes problematic. 
description: All data are provided in CSV formats and should be usable in
 basic programs, like Excel.