Identification of Winter Flounder (Pseudopleuronectes americanus) Estuarine Spawning Habitat and Factors Influencing Egg and Larval Distributions

A long-term (2002–2011), spatially robust, ichthyoplankton sampling program conducted in the New York/New Jersey Harbor produced 3,033 epibenthic samples from which the relationships between winter flounder egg and larval distributions and environmental parameters were examined. Variations in water temperature, sediment characteristics, and tidal phase were all significantly associated with egg distributions. Inferences about spawning habitats were based on the presence of early-stage eggs (ES1 and ES2). In the Lower Bay (LB), these habitats were primarily non-channel and characterized by more sandy substrates, averaging 96.5 % sand, 2.3 % silt/clay, 0.2 % total organic carbon (TOC), and shallower water (average depths of 5.3 m) compared to LB non-channel stations without ES1 and ES2 eggs (50.2 % sand, 42.0 % silt/clay, 2.1 % TOC, and 7.9 m depths). Occurrences of all stages of eggs in channels were associated with strong tides and severe cold winter water temperatures. These conditions increase the probability of egg transport from shallow spawning sites through increased vertical mixing (strong tides) and delayed development that prolongs the risk of displacement (cold temperatures). Yolk-sac (YS) and Stage-2 larvae were smaller in 2010 when spring water temperatures were highest. Overall, YS larval size decreased with warmer winters (cumulative degree-days for the month preceding peak YS larval collections, r ²?=?0.82, p?<?0.05). In all years, YS larvae collected in LB were smaller and Stage-3 larvae collected in channels were larger and possibly older than those from non-channel habitat. Because estuarine winter flounder populations are highly localized, adverse effects experienced during egg and larval stages are likely to propagate resulting in detrimental consequences for the year class in the natal estuary.     

The geology of the Oceanographer Transform: The transform domain

Three dives in submersible ALVIN and four deep-towed camera lowerings have been made along the transform valley of the Oceanographer Transform. These data constrain our understanding of the processes that create and shape the distinctive morphology that is characteristic of slowly slipping ridge-transform-ridge (RTR) plate boundaries. Our data suggest that the locus of strike-slip tectonism, called the transform fault zone (TFZ), is confined to a narrow swath (<4 km) that is centered along the axis of maximum depth. The TFZ is flanked by the inward facing slopes of the transform valley. The lower portions of the valley walls are characterized by broad sloping exposures of undisrupted sediment but at higher elevations the walls are made up of inward facing scarps and terraces of variable dimensions. Although the scarps have been badly degraded by mass wasting, there is no evidence to suggest that these scarps have accommodated significant amounts of strike-slip motion. Plutonic and ultramafic rocks are exposed on these scarps and the occurrence of this diverse assemblage on small-throw faults indicates that the crust is thin and/or discontinuous in this environment. We suggest that this complex igneous assemblage is the product of anomalous accretionary processes that are characteristic of slowly-slipping RTR plate boundaries.     

Structure and topography of the Siqueiros transform fault system: Evidence for the development of intra-transform spreading centers

The Siqueiros transform fault system, which offsets the East Pacific Rise between 8°20N–8°30N, has been mapped with the Sea MARC II sonar system and is found to consist of four intra-transform spreading centers and five strike-slip faults. The bathymetric and side-looking sonar data define the total width of the transform domain to be 20km. The transform domain includes prominent topographic features that are related to either seafloor spreading processes at the short spreading centers or shearing along the bounding faults. The spreading axes and the seafloor on the flanks of each small spreading center comprise morphological and structural features which suggest that the two western spreading centers are older than the eastern spreading centers. Structural data for the Clipperton, Orozco and Siqueiros transforms, indicate that the relative plate motion geometry of the Pacific-Cocos plate boundary has been stable for the past 1.5 Ma. Because the seafloor spreading fabric on the flanks of the western spreading centers is 500 000 years old and parallels the present EPR abyssal hill trend (350°) we conclude that a small change in plate motion was not the cause for intra-transform spreading center development in Siqueiros. We suggest that the impetus for the development of intra-transform spreading centers along the Siqueiros transform system was provided by the interaction of small melt anomalies in the mantle (SMAM) with deepseated, throughgoing lithospheric fractures within the shear zone. Initially, eruption sites may have been preferentially located along strike-slip faults and/or along cross-faults that eventually developed into pull-apart basins. Spreading centers C and D in the eastern portion of Siqueiros are in this initial pull-apart stage. Continued intrusion and volcanism along a short ridge within a pull-apart basin may lead to the formation of a stable, small intra-transform spreading center that creates a narrow swath of ridge-parallel structures within the transform domain. The morphology and structure of the axes and flanks of spreading centers A and B in the western and central portion of Siqueiros reflect this type of evolution and suggest that magmatism associated with these intra-transform spreading centers has been active for the past 0.5–1.0 Ma.