Patterns in vertical distribution and their potential effects on transport of larval benthic invertebrates in a shallow embayment

Abstract

Measurements of larval vertical distributions at high temporal and spatial resolutions as well as larval behavioural responses to environmental characteristics are needed to parameterize bio-physical models of larval dispersal or transport. We studied larval vertical distribution for 7 taxonomic groups (gastropods, bivalves, polychaetes, bryozoans, asteroids, carideans and brachyurans), with different morphology, swimming abilities and life-history strategies, and examined whether these vary with physical or biological factors and periodic cycles (diel period and tidal state) in the field. Using a pump, we collected plankton samples at 6 depths (3, 6, 9, 12, 18 and 24 m), over a 36 and a 26 h period. Temperature, salinity, fluorescence and current velocity were measured concurrently. Larval vertical distribution varied among taxonomic groups, but 4 patterns could be distinguished: (1) larvae exclusively in the mixed layer (asteroids), (2) larvae predominantly below the thermocline, halocline and pycnocline (gastropods, bivalves, polychaetes), (3) larvae associated predominantly with the fluorescence maximum (bryozoans and carideans) and (4) larval distribution varying dielly (gastropods, polychaetes, carideans and brachyurans). Based on flow velocities and depending on distribution, asteroid larvae were likely to be transported farther than those of bryozoans and carideans, while direction and magnitude of transport varied for the other larvae. For most taxonomic groups, behaviour observed in the field agreed with measured laboratory responses to relevant cues. For asteroids and bivalves, simple beha vioural parameters can be generated that can be utilized to improve the accuracy of biophysical models.KEY WORDS: Vertical migration · Temperature · Fluorescence · Diel period · Water column structure · Larval behavior · Larval transport · Biophysical model parameters Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 469: [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] 2012 Horizontal currents can ad vect larvae of coastal species offshore, leading to failure of recruitment to nearshore habitats (Shanks 1995).Meroplankton are capable of movement against vertical currents, as their swimming or sinking speeds are greater than weak vertical current velocities (0.1 cm s −1 ) (Chia et al. 1984, Metaxas 2001. Larval movement between layers of different velocities can alter the horizontal direction and magnitude of larval transport and dispersal (DiBacco et al. 2001, Metaxas 2001. Through changes in buoyancy or propulsion by ciliary or muscular activity, some crustacean and bivalve larvae can cover large distances vertically, in some cases many times a day (Cronin & Forward 1986, dos Santos et al. 2008. Larval movement can possibly be triggered through an innate behavioural response to physical and/or chemical stimuli. Consequently, sensory detection of the environment can potentially affect larval direction of movement and/or swimming be haviour (...

Tidal shifts in the vertical distribution of bivalve larvae: Vertical advection vs. active behavior

“…Within our study region, tidal shifts in the vertical distribution of larvae appear to occur reliably and repeatedly (Fig. ) and involve vertical movement below the pycnocline, as has been reported in other studies (Raby et al ; Lloyd et al ). However, distributions centered above the pycnocline have also been noted in some regions (Dobretsov and Miron ).…”

Constrained nearshore larval distributions and thermal stratification

“…Cues that might be used to make vertical changes in distribution include temperature. For example, gastropods, bivalves, and polychaetes in St. George's Bay, Nova Scotia, Canada, remained below the thermocline and pycnocline, and variations in their abundances were associated with temperature (Lloyd et al 2012). Strong thermoclines can also restrict vertical distributions.…”

Delivery of zooplankton to the surf zone during strong internal tidal forcing and onshore winds in Baja California

“…The relationship detected between tidal height and abundance of bryozoans suggests that the incursion of water onto the shore by the tide, or some other factor related to the tide, may play a role in their delivery to shore. The different results found for different sites also underscores the importance of vertical position of zooplankters in determining which transport mechanisms they are exposed to (Paris & Cowen 2004, Hare et al 2005, Lloyd et al 2012a. Larvae cross the surf zone by several means, depending on their vertical distribution.…”