Influences of Salinity and Light Availability on Abundance and Distribution of Tidal Freshwater and Oligohaline Submersed Aquatic Vegetation

Abstract

Submersed aquatic vegetation (SAV) communities have undergone declines worldwide, exposing them to invasions from non-native species. Over the past decade, the invasive species Hydrilla verticillata has been documented in several tributaries of the lower Chesapeake Bay, Virginia. We used annual aerial mapping surveys from 1998 to 2007, integrated with spatial analyses of water quality data, to analyze the patterns and rates of change of a H. verticillata-dominated SAV community and relate them to varying salinity and light conditions. Periods of declining SAV coverage corresponded to periods where salinities exceeded 7 and early growing season (April to May) Secchi depths were <0.4 m. Increases were driven by the expansion of H. verticillata along with several other species into the upper estuary, where some areas experienced an 80% increase in cover. Field investigations revealed H. verticillata dominance to be limited to the upper estuary where total suspended solid concentrations during the early growing season were <15 mg l −1 and salinity remained <3. The effect of poor early growing season water clarity on annual SAV growth highlights the importance of water quality during this critical life stage. Periods of low clarity combined with periodic salinity intrusions may limit the dominance of H. verticillata in these types of estuarine systems. This study shows the importance of the use of these types of biologically relevant episodic events to supplement seasonal habitat requirements and also provides evidence for the potential important role of invasive species in SAV community recovery.

Competition in a changing world: invasive aquatic plant is limited by saltwater encroachment

“…Our findings suggest that hydrilla invasiveness may be weakened by saltwater intrusion, particularly if this exposure occurs in conjunction with a seasonal period of low production. This hypothesis is supported by observations in the lower Chesapeake Bay system, where periodic salinity intrusions combined with low productivity conditions during the critical early growing season led to subsequent declines in hydrilla dominance (Shields et al, 2012). However, if competitive interactions prior to a salinity intrusion resulted in hydrilla vastly outcompeting wild celery shoots for resources, both species may become overstressed by salinity exposure and decline.…”

“…A caveat to our speculation is that both the hydrilla and wild celery shoots used in this study originated from lakes that do not experience saltwater intrusion, and that plants from habitats that are subjected to occasional salinity encroachment may have a higher salinity tolerance threshold. Unlike the consistent reports of salinity limitations for the North American invasive strain of hydrilla (Haller et al, 1974; Hillmann et al, 2019; Shields et al, 2012; Twilley & Barko, 1990), reported tolerance thresholds for wild celery have been highly variable in the literature, ranging from as low as approximately 6.6 (Haller et al, 1974) up to salinities greater than 15 (Kraemer et al, 1999), suggesting wide physiological variability in salinity tolerance thresholds among wild celery populations. In this study, salinity treatments for both wild celery and hydrilla showed declines in production and shoot size in addition to shifted biomass allocation strategies, suggesting impacts to production capabilities or soluble carbohydrate reserves (Kraemer et al, 1999; Madsen & Owens, 1998).…”

Natural variation on whole‐plant form in the wild is influenced by multivariate soil nutrient characteristics: natural selection acts on root traits

“…A. Middleton, personal observation). Thus, hurricane salinity intrusion could contribute to long‐term change (Hayden et al, 1991) in coastal wetlands, especially because of the differential effects of salinity on regeneration (this study, Baldwin et al, 1996) and vegetation composition (Williams, 1993; Conner, 1995; Stanturf et al, 2007; Hoeppner et al, 2008; Shields et al, 2011; Werner et al, 2013).…”

Effects of salinity and flooding on post‐hurricane regeneration potential in coastal wetland vegetation

“…A. Middleton, personal observation). Thus, hurricane salinity intrusion could contribute to long-term change ( Hayden et al, 1991 ) in coastal wetlands, especially because of the diff erential eff ects of salinity on regeneration (this study, Baldwin et al, 1996 ) and vegetation composition ( Williams, 1993 ;Conner, 1995 ;Stanturf et al, 2007 ;Hoeppner et al, 2008 ;Shields et al, 2011 ;Werner et al, 2013 ).…”