Michael H. Doall

Climate change, precipitation and impacts on an estuarine refuge from disease.

Background Oysters play important roles in estuarine ecosystems but have suffered recently due to overfishing, pollution, and habitat loss. A tradeoff between growth rate and disease prevalence as a function of salinity makes the estuarine salinity transition of special concern for oyster survival and restoration. Estuarine salinity varies with discharge, so increases or decreases in precipitation with climate change may shift regions of low salinity and disease refuge away from optimal oyster bottom habitat, negatively impacting reproduction and survival. Temperature is an additional factor for oyster survival, and recent temperature increases have increased vulnerability to disease in higher salinity regions. Methodology/Principal Findings We examined growth, reproduction, and survival of oysters in the New York Harbor-Hudson River region, focusing on a low-salinity refuge in the estuary. Observations were during two years when rainfall was above average and comparable to projected future increases in precipitation in the region and a past period of about 15 years with high precipitation. We found a clear tradeoff between oyster growth and vulnerability to disease. Oysters survived well when exposed to intermediate salinities during two summers (2008, 2010) with moderate discharge conditions. However, increased precipitation and discharge in 2009 reduced salinities in the region with suitable benthic habitat, greatly increasing oyster mortality. To evaluate the estuarine conditions over longer periods, we applied a numerical model of the Hudson to simulate salinities over the past century. Model results suggest that much of the region with suitable benthic habitat that historically had been a low salinity refuge region may be vulnerable to higher mortality under projected increases in precipitation and discharge. Conclusions/Significance Predicted increases in precipitation in the northeastern United States due to climate change may lower salinities past important thresholds for oyster survival in estuarine regions with appropriate substrate, potentially disrupting metapopulation dynamics and impeding oyster restoration efforts, especially in the Hudson estuary where a large basin constitutes an excellent refuge from disease.

Effects of Zooplankton Grazing on Phytoplankton Size-Structure and Biomass in the Lower Hudson River Estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various14C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m−2 h−1/mg C produced m−2 h−1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.

Evaluating Northern Quahog (= Hard Clam, Mercenaria mercenaria L.) Restoration: Are Transplanted Clams Spawning and Reconditioning

Abstract Spawner sanctuaries, harvest-free areas planted with high densities of adult clams, are currently being used to restore self-sustaining populations of Mercenaria mercenaria (L.) to Great South Bay, New York. To evaluate and guide this restoration, we monitored the condition and spawning of clams transplanted from two source locations in Long Island Sound since April 2004. Transplanted clams were in relatively high condition and gonad ripeness at time of transplant, spawned the first spring and/or summer after transplant, reconditioned and spawned in subsequent years, but rarely reconditioned to as high of levels as when they were first transplanted. All populations exhibited similar annual patterns of condition and gonad ripeness: both peaked in mid to late spring, dropped steeply through summer with spawning, and they were lowest in fall. In some years condition increased during fall, and the higher the condition attained by the end of fall (mid-December) the greater the peak in condition the following spring. Across years and populations, condition at the end of fall explained ∼89% of the variance in spring peak condition. Consistent differences in condition through time among some populations suggested that location within the bay as well as clam size impact condition. Because of interannual and locational variability, long-term monitoring of this long-lived species is essential for determining factors affecting condition and reproduction, and the ultimate restoration of sustainable hard clam populations.

RESTORATION OF CRASSOSTREA VIRGINICA (GMELIN) TO THE HUDSON RIVER, USA: A SPATIOTEMPORAL MODELING APPROACH

ABSTRACT As a result of its historical abundance and ecological significance, the eastern oyster, Crassostrea virginica, has been identified as a primary restoration target for the Hudson River-New York Harbor region. Prior to any large-scale restoration investments, a spatial assessment has been made to characterize the regions potential for hosting restored oyster populations. Using existing geographic data of the physical attributes of the river, a GIS-based restoration suitability index has been developed with the goal of identifying specific areas that hold a greater probability for success in oyster restoration. The results show that much of the rivers restoration potential is initially limited by the physical environment, depth, and sediment type, and is reduced further by the salinity distribution. The results from this model should be used as a preliminary guide to focus future restoration efforts within the lower Hudson River and New York Harbor area as well as to investigate possible changes to the restoration potential with changing salinities brought on by regional climate change and hydrodynamic alterations.

GROWTH AND MORTALITY PATTERNS OF THE EASTERN OYSTER CRASSOSTREA VIRGINICA IN IMPACTED WATERS IN COASTAL WATERS IN NEW YORK, USA

ABSTRACT We monitored cage-based populations of the eastern oyster Crassostrea virginica in coastal waters of New York Harbor in 2 phases of sampling, 1 with localities spread out over the New York—New Jersey Harbor area (started 2008) and another with 3 localities within Jamaica Bay (started 2010), all impacted by high nitrogen input, low dissolved oxygen, but over a water quality gradient. Patterns of growth, mortality, condition, and disease were compared with a clean—water site in Shelter Island, NY, sampled in parallel with both sampling phases. In both studies, oyster mortality in the urban sites increased dramatically during and after the second summer growth season. Mortality also increased at the same time period at the cleanwater site, but to a much smaller degree. One instance of high mortality in the Lower Hudson was caused by MSX; but, otherwise, no known diseases were identified as the main cause of the sudden mortality increases. Our results suggest that a general effect of reduced water quality had a cumulative effect on the New York Harbor-emplaced oysters, which culminated in high mortality, mainly at the end of the second summer growing season. Despite the increased mortality, other factors such as soft tissue growth and reproduction were not reduced in the harbor sites relative to the clean-water control site. The vulnerability of oysters grown in impacted waters may have to be factored in attempts to restore oysters to impacted harbor waters.

Testing the Accuracy of Morphological Identification of Northern Quahog Larvae

Abstract Bivalve larvae in mixed samples collected from the field have traditionally been identified through morphological differences among species. It is difficult, however, to use this method accurately because of overlapping size ranges and similar shapes of the larvae of many species. We used the molecular technique developed by Hare et al. (2000) to test the accuracy of morphological identification of Mercenaria mercenaria (L.) larvae from plankton samples collected from the Great South Bay and Coecles Harbor on Long Island, New York. We found that morphology is unreliable as the only means of identification for bivalve larvae in a mixed field sample, and a very high false positive rate of identification of M. mercenaria (100% of 71 larvae were misidentified). Morphological characteristics may be used to eliminate larvae from a field plankton sample, as the false negative rate for M. mercenaria was only 1.4% (n = 140). To determine larval bivalve densities accurately, other techniques in addition to those based on morphological characteristics, such as the molecular technique used in this study, must be used.