Effects of Altered Shorelines on Macrofauna and Diel-Cycling Hypoxia in Tidal Tributaries of Delaware Bay and Delaware Coastal Bays
University of Delaware
The physical structure of estuarine habitats has been, and continues to be modified by human activity. Understanding how the biophysical structure of shoreline affects the functional value and habitat quality of estuaries for shore-zone biota is important in determining impacts of shoreline modification on estuarine systems. The Mid-Atlantic region has seen a decrease in the area of Spartina alterniflora (smooth cordgrass) within estuaries during the past few decades, while invasive Phragmites australis (common reed) and several types of shoreline hardening including bulkhead, riprap and riprap-sill structures have become more common. These hardened shorelines and invasive grass species have caused a change in the ecological character of the intertidal zone, particularly in urbanized watersheds. This research seeks to understand how changes in shoreline structure affect the ecological character of tidal marsh creeks and basins along Delaware Bay and Delaware Coastal Bays through estuarine fish and crab assemblages, tributary water quality, and fish spawning activity. In the first part of this study, I used quantitative sampling to examine the spatiotemporal use of estuarine systems by fish and blue crabs in association with environmental characteristics of the habitat, in particular shoreline type and water quality. Fish and blue crabs were collected from the shore-zone of five shoreline types, S. alterniflora marsh, P. australis marsh, sandy beach, riprap, and bulkhead from August through September 2009 and June through September 2010. Over the two-year study, in total 102,343 individuals in 28 species of fish and 3,607 blue crabs were collected in the shore-zone of Indian River and Pepper Creek. The greatest abundances of fish were collected from the shore-zone of native S. alterniflora marsh, while the greatest abundances of blue crabs were caught in the shore-zone of P. australis marsh. Individual species of abundant estuarine fish had unique relationships with hardened and unhardened shorelines. Total fish abundance was greatest when dissolved oxygen (DO) was above the EPA dissolved oxygen criterion of 2.3 mg O2/l for the survival of aquatic organisms, though this relationship differed among fish species and shoreline type. Minimal associations were found between fish and blue crab abundance and other environmental variables including water temperature, watershed land use, and watershed impervious surface area. Overall fish community assemblages did not differ in any perceptible pattern among shoreline types. Fish assemblages at all sampling stations were more idiosyncratic than coupled based on shoreline type. In the second part of this study, I used quantitative sampling to examine the spatiotemporal use of estuarine systems by fish and blue crabs in association with riprap-sill. Wetland managers generally consider riprap-sill structures (a type of ‘living shoreline’ consisting of a rock sill placed low in the intertidal zone with native vegetation planted between the sill and shore) to be more ecologically sound than riprap for shoreline stabilization in estuaries. However, little research has been conducted comparing the macrofauna associated with these structures with that inhabiting the more traditionally applied riprap. Fish and blue crab abundance and diversity were compared at riprap-sill shoreline, riprap shoreline, and shoreline fringed with smooth cordgrass (Spartina alterniflora) marsh in the Delaware Coastal Bays, USA, from June through September 2010. Seining was conducted to quantitatively sample the shore-zone and shallow subtidal regions, and minnow traps were used to determine presence/absence of fishes in the upper intertidal zone of each shoreline type. In total, 9777 fishes and 548 blue crabs were collected from the shore-zone of all three shoreline types during seining. The greatest abundance and density of fishes were caught along the S. alterniflora shoreline. Species densities and composition were generally more similar between the S. alterniflora and the riprap-sill shoreline than were the riprap-sill and traditional riprap shoreline. This study demonstrates that although S. alterniflora shoreline is inhabited by the highest abundance and density of fishes, riprap-sill structure provides a better alternative than does traditional riprap in terms of abundance and diversity of shore-zone estuarine fish and blue crabs. In the third part of this study, multi-parameter water quality meters were positioned in the subtidal zone immediately adjacent to each of five shoreline types, S. alterniflora marsh, P. australis marsh, sandy beach, riprap, and bulkhead, within four tributary regions of Indian River and Pepper Creek. Multi-parameter sondes were alternated among each tributary region once at 2 week intervals to record differences in water temperature and DO concentrations during the summer of 2010. Measurements were recorded when tributaries were experiencing severe diel-cycling hypoxia (July 15th-September 9th, 2010). Negligible differences were found in water temperature among shoreline types. The subtidal area directly adjacent to S. alterniflora shorelines was found to experience less severe diel-cycling hypoxia than other shoreline types in Indian River and Pepper Creek. This was particularly evident between the hours of 0200 and 1000, when DO concentrations are typically at their lowest, wherein water adjacent to S. alterniflora shorelines had consistently greater DO concentrations than did water adjacent to other shoreline types. In the fourth part of this study, I examined how Atlantic silverside (Menidia menidia) utilized six different shoreline types (S. alterniflora marsh, P. australis marsh, sandy beach, riprap-sill, riprap, bulkhead) for egg deposition. Atlantic silverside is among the most abundant forage fish species in Mid-Atlantic estuaries, and is an important prey for piscivores such as striped bass, Atlantic mackerel, bluefish, and others. Atlantic silverside eggs are demersal, adhesive, and are laid in estuarine intertidal zones. From April 14th to June 10th, 2010, egg deposition of Atlantic silverside, was measured daily at several sites near Roosevelt Inlet, Delaware, close to the mouth of Delaware Bay. Substrates utilized for egg attachment were noted at each shoreline type. Air/water temperature was recorded every 15 minutes at high, mid, and low intertidal elevations using iButton thermochrons. Over 3,000,000 eggs were collected during 50 sampling days. Eggs were deposited at all six shoreline types, with >93% of eggs collected from S. alterniflora shorelines. Choice of substrate for egg attachment was similar across shoreline types with >91% of eggs collected from filaments of the green alga Enteromorpha spp., a disproportionately high utilization rate in comparison with Enteromorpha spp.’s relative coverage. This study demonstrates that S. alterniflora shoreline, in association with Enteromorpha spp., is preferred spawning habitat for M. menidia, and that hardened shorelines and shorelines inhabited by P. australis support substantially reduced egg densities.