Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge

Abstract

Groundwater discharge is a common phenomenon along sandy coasts. At Cape Henlopen, Delaware, USA, it creates low porewater salinity in spatially heterogeneous patterns over 1 to 10 m horizontally. In the present study, porewater salinity and macrofaunal communities were sampled along intertidal transects in summer and spring. We consistently observed high spatial variability in porewater salinity at 10 cm sediment depth, with changes up to 15.8 m -1. Community composition and species abundance differed between the seasons, but, in each season, communities associated with low porewater salinity (<15) differed from those associated with higher porewater salinity (15 to 24 or > 24). In general, species from a diverse pool of sand flat species gradually became absent as salinity decreased. These local community changes correlated with porewater salinity could compromise the interpretation of various benthic indices that are based on watercolumn salinity, which, at this site, remains around 28. In most samples, the polychaete Marenzelleria viridis was numerically dominant (up to 10 000 m -2 ) in burrows up to 50 cm deep. Both in the field and in cores constructed in the laboratory, we found that the number of worm burrows was highly correlated with hydraulic conductivity of the sediment. We propose that burrowing worms (especially M. viridis) at this groundwater discharge site act as hydraulic ecosystem engineers by means of their burrowing and tube building. In general, there appear to be strong physical-biological interactions among the distribution of organisms, their infaunal behavior, the hydrological properties of the sediment and the magnitude of groundwater discharge that may contribute to infaunal zonation on intertidal sand flats.

Restricted ranges in physical factors may constitute subtle stressors for estuarine biota

“…6B), leaving relatively coarse substrates at all tidal elevations. Hydraulic conductivity of such coarse beaches is high, meaning that the marine water table drains relatively rapidly at low tide compared with mud beaches (Dale and Miller, 2008). The level of the water table, in turn, has significant impacts on factors important to infauna such as temperature and moisture content (Dale and Miller, 2008;Jackson et al, 2008;Li et al, 2006).…”

“…Hydraulic conductivity of such coarse beaches is high, meaning that the marine water table drains relatively rapidly at low tide compared with mud beaches (Dale and Miller, 2008). The level of the water table, in turn, has significant impacts on factors important to infauna such as temperature and moisture content (Dale and Miller, 2008;Jackson et al, 2008;Li et al, 2006). In our study, elevation was constant among beaches but the moisture content clearly varied, as evidenced by the rate of filling of holes dug for porewater measurements (one of the Brown beaches was usually dry when sampled, for example); this variation could be due to hydraulic conductivity of the beach and/or to the amount of Potentially important factors that were not included in our analyses of the drivers of diversity patterns are levels of anthropogenic disturbance (e.g., bulkheading of the shore; see Seitz et al 2006) and biotic factors such as nutrient sources, dispersal, recruitment, competition, and predation.…”

“…These issues have received attention in the growing geomorphological literature about the shoreline water table, especially on sandy beaches (Jackson et al, 2002;Li et al, 2006), but have seldom been related directly to their impacts on beach biota. Several recent studies have shown that local groundwater characteristics, especially low-salinity seeps, can significantly affect flora and fauna (Dale and Miller, 2008;Rumrill and Sowers, 2008;Zipperle and Reise, 2005) and thus behavior of predators such as birds (Rosa et 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Dethier-16 16 al., 2007). The shorelines of Puget Sound are mostly backed by bluffs of glacial deposits (as illustrated in Jackson et al 2002); erosion of these leads to beaches that are a mix of many sediment types.…”

Differences in microphytobenthos and macrofaunal abundances associated with groundwater discharge in the intertidal zone

“…This approach is an important advancement in studies on the effects of intertidal groundwater seepage, which in the past either focused on the algal or the macrofaunal communities. It is likely that the benthic intertidal community creates feedback loops that interact with the 'original' impact of the groundwater seepage: For example, increased microphytobenthos biomass may create larger grazing pressure, or seepage areas could attract burrowing organisms, which enhance the hydraulic conductivity even further (Dale & Miller 2008). …”

The Three Scales of Submarine Groundwater Flow and Discharge across Passive Continental Margins

“…Many recent studies have linked submarine groundwater discharge to the occurrence and health of important coastal ecosystems and fauna including, at the nearshore scale, salt marshes (Valiela et al 1978;Krest et al 2000;Charette 2007), mangroves (McGowan and Martin 2007), polychaetes (Dale and Miller 2008), bivalves (Taniguchi et al 2008), and microbial pathogen assemblages (Boehm et al 2004); and, at the embayment and shelf scales, coral reefs (Gagan et al 2002;Paytan et al 2006;Street et al 2008), and fish (Culter 2006). The locations and chemistry of discharging water can have important impacts on benthic coastal ecosystems (Johannes 1980), particularly at the near-shore scale, even to the point of controlling their formation, their evolution, and the behavior of organisms living within them.…”