Jesús Pineda

Predictable upwelling and the shoreward transport of planktonic larvae by internal tidal bores.

Internal tidal bores have a crucial role in the transport of drifting larvae to marine nearshore populations, a key factor in structuring benthic communities. Shoreward transport of larvae and abrupt surface temperature drops lasting days can be explained by invoking the advection of subsurface cold water to the shore by internal tidal bores. This process is predictable within the lunar cycle and brings deep water to the surface (upwelling) in a direction perpendicular to the coastline.

Internal tidal bores in the nearshore: warm-water fronts, seaward gravity currents and the onshore transport of neustonic larvae

Nearshore temperature fluctuations are associated with energetic cross-shore two-way flows that influence the onshore transport of neustonic larvae. Water temperature near the surface and bottom at two nearshore stations off southern California (6 and 15 m water depth, respectively) can drop sharply and subsequently rise. Two or more consecutive drops and rises can occur at diurnal or semidiurnal periodicities. The temperature increases may be accompanied by energetic seaward bottom currents together with sharp-edged warm-water fronts. (Warm-water fronts are defined here as linear seasurface features dividing parcels of water of different temperature.) Shoreward-moving surface fronts divided bodies of water of different surface temperature, where the coldest water body was inshore. Fronts disappeared at (or close to) the surf zone. The sharp drops in water temperature are interpreted as the onshore advection of subsurface water by large internal tidal bores, and it is concluded that the sudden increases in temperature and cross-shore advection are epiphenomena of internal tidal bores. Internal tidal bores have been invoked previously to explain the onshore transport of water-column larvae. This study tests the hypothesis that shoreward surface flow, an epiphenomenon of internal tidal bores, transports neustonic larvae in warm-water fronts. Five warm-water fronts were sampled in shallow water (about 6 m) for temperature and fish and crab larvae in June-July 1992. These larvae were more abundant in fronts than in parcels of water preceding or following the front. Peaks in larval abundance were accompanied by a sharp rise in temperature, in itself evidence for onshore transport of surface water. It is concluded that both warm-water fronts and internal tidal bores play a key role in the exchange across the shelf of material and water properties, and that internal tidal-bore phenomena explain well the transport of both water-column and neustonic larvae in different habitats.

Complexity and simplification in understanding recruitment in benthic populations

Research of complex systems and problems, entities with many dependencies, is often reductionist. The reductionist approach splits systems or problems into different components, and then addresses these components one by one. This approach has been used in the study of recruitment and population dynamics of marine benthic (bottom-dwelling) species. Another approach examines benthic population dynamics by looking at a small set of processes. This approach is statistical or model-oriented. Simplified approaches identify “macroecological” patterns or attempt to identify and model the essential, “first-order” elements of the system. The complexity of the recruitment and population dynamics problems stems from the number of processes that can potentially influence benthic populations, including (1) larval pool dynamics, (2) larval transport, (3) settlement, and (4) post-settlement biotic and abiotic processes, and larval production. Moreover, these processes are non-linear, some interact, and they may operate on disparate scales. This contribution discusses reductionist and simplified approaches to study benthic recruitment and population dynamics of bottom-dwelling marine invertebrates. We first address complexity in two processes known to influence recruitment, larval transport, and post-settlement survival to reproduction, and discuss the difficulty in understanding recruitment by looking at relevant processes individually and in isolation. We then address the simplified approach, which reduces the number of processes and makes the problem manageable. We discuss how simplifications and “broad-brush first-order approaches” may muddle our understanding of recruitment. Lack of empirical determination of the fundamental processes often results in mistaken inferences, and processes and parameters used in some models can bias our view of processes influencing recruitment. We conclude with a discussion on how to reconcile complex and simplified approaches. Although it appears impossible to achieve a full mechanistic understanding of recruitment by studying all components of the problem in isolation, we suggest that knowledge of these components is essential for simplifying and understanding the system beyond probabilistic description and modeling.

Bathymetric species-diversity patterns and boundary constraints on vertical range distributions

Abstract A system where a set of species is randomly distributed within two spatial boundaries produces parabolic species diversity patterns, where diversity is lowest in the vicinity of the boundaries, and highest in the mid-zone between the boundaries. We demonstrate this phenomenon using simulated data. We tested whether parabolic depth trends in species diversity (Rex, 1981, 1983) can be explained by invoking only this phenomenon. We analyzed data sets for northwest-Atlantic gastropods and polychaetes previously used to document parabolic diversity patterns. Data sets are matrices of species’ abundance where rows index depth, and columns species. Simulations where species vectors were randomly rearranged within the shallowest and deepest stations generally produced parabolic diversity patterns, with higher diversity at intermediate zones, and lowest diversity closest to the shallowest and deepest stations. For the gastropods, the location of the peaks for the observed and randomly rearranged taxa coincided at similar depths. Random rearrangements, however, did not match the original patterns in curvature (peakedness of the diversity curve) or in magnitude (height of the diversity peak). Highest values were for observed taxa, implying that the original distribution of species is highly non-random and that other factors not assumed in the simulation influence bathymetric species-diversity patterns. For the polychaetes, randomly rearranged data sets matched the magnitude of the observed data set. The original parabolic diversity curve, however, peaked at a shallower location than the random rearrangements, and the magnitude of the peak was higher for the observed taxa. Overall, we find that the random rearrangements cannot explain most characteristics of the parabolic diversity patterns of gastropods and polychaetes. We also explored the influence of species with large vertical range in influencing parabolic species diversity patterns. Removal “experiments”, where a portion of the species with the largest vertical range was removed, also removed parabolic bathymetric diversity patterns, for both observed and simulated taxa, suggesting that species with large vertical range are disproportionally important in determining such patterns.

Temperature, stratification and barnacle larval settlement in two Californian sites

Barnacle settlement was monitored in two sites 100 km apart along the coast of Alta and Baja California. In five periods of observations completed between 1991 and 1996, Chthamalus spp., Pollicipes polymerus, and Balanus glandula settlement was consistently higher in the northern site, La Jolla (LJ), than in the southern site, La Salina (LS). For Chthamalus, the most abundant settler, settlement was higher in LJ in 58 out of 60 paired dates, by a mean factor of 141. In 1996, time series of temperature in about 15 m of water showed that the stratification was 72% higher, on average, and that the thermocline was shallower in LJ than in LS. Spectra of temperature showed that internal motions of tidal and higher frequencies were more energetic and closer to the surface in LJ compared to LS. In LJ changes in settlement were positively correlated with changes in stratification. These results suggest that high-frequency internal motions are important in the onshore transport of larvae. Low-frequency cooling events recorded in LJ apparently caused the energetic semidiurnal temperature variability to migrate from the bottom towards the surface, leading to the surface manifestation of the internal tide and surface internal tidal bores, which indicates that the surface nearshore bores occur in response to the shallowing of the thermocline. Tidal and higher frequency internal motions were more energetic when the thermocline was shallow during the low-frequency cooling events, than when it was deep and relatively weak during ordinary conditions. The major cooling event in LJ correlated with the local wind, suggesting local wind-driven upwelling. On the other hand, correlation of LS temperature with LJ temperature, winds, and sea level suggest propagation from the South. These results suggest that the low-frequency drops in temperature that modulate the nearshore internal tidal bores are caused by a combination of the local wind and events that propagate poleward, possibly as coastally trapped waves.

Observations of the thermal environment on Red Sea platform reefs: a heat budget analysis

Hydrographic measurements were collected on nine offshore reef platforms in the eastern Red Sea shelf region, north of Jeddah, Saudi Arabia. The data were analyzed for spatial and temporal patterns of temperature variation, and a simple heat budget analysis was performed with the goal of advancing our understanding of the physical processes that control temperature variability on the reef. In 2009 and 2010, temperature variability on Red Sea reef platforms was dominated by diurnal variability. The daily temperature range on the reefs, at times, exceeded 5°C—as large as the annual range of water temperature on the shelf. Additionally, our observations reveal the proximity of distinct thermal microclimates within the bounds of one reef platform. Circulation on the reef flat is largely wave driven. The greatest diurnal variation in water temperature occurs in the center of larger reef flats and on reefs protected from direct wave forcing, while smaller knolls or sites on the edges of the reef flat tend to experience less diurnal temperature variability. We found that both the temporal and spatial variability in water temperature on the reef platforms is well predicted by a heat budget model that includes the transfer of heat at the air–water interface and the advection of heat by currents flowing over the reef. Using this simple model, we predicted the temperature across three different reefs to within 0.4°C on the outer shelf using only information about bathymetry, surface heat flux, and offshore wave conditions.

An internal tidal bore regime at nearshore stations along western U.S.A.: Predictable upwelling within the lunar cycle

Nearshore upwelling due to predictable large internal bores may be a widespread phenomenon along the west coast of North America. Internal tidal bores (breaking internal tidal waves) cause drops in surface water temperature that last for 2–9 days. Negative surface water temperature anomalies (anomaly = daily datum minus day-of-the-year average) often reflect large internal tidal bores. These anomalies are predictable within the lunar cycle in spring and summer (but not in winter and fall) at a Southern California shore-station, the Scripps Institution of Oceanography Pier [Pineda (1991) Science, 253, 548–551]. The internal tidal bore hypothesis has been invoked to predict and explain this result: anomalies are predictable (on average) within the lunar cycle because of a lunar or spring-to-neap cycle in internal bore activity. The anomalies are more predictable in spring and summer than in fall and winter because internal waves are most energetic when the water column is well stratified. Long time series (18–64 years) of surface water temperature from 10 U.S. west coast shore stations from Oceanside in southern California to Neah Bay in Washington, were analyzed to evaluate the generality of the Scripps results. Nine stations showed that temperature anomalies are predictable on average within the lunar cycle in spring or summer, but not in fall and winter. There is high variability in the magnitude and variance of the average anomaly among stations. In general, for spring and summer, the most negative anomalies tend to occur on days 7–12 ( - ) and 19–24 ( - ) of the lunar cycle (day one being day after the new moon ). Few most-negative-anomalies occurred around new moon or full moon. The Farallon Islands station showed a more random distribution of anomalies within the lunar cycle during spring and summer.

Plankton accumulation and transport in propagating nonlinear internal fronts

Accumulation and transport of plankton in fronts propagating across-shore is a process of considerable ecological importance for many inhabitants of the littoral zone, since it links the offshore larval pool with the juvenile and adult inshore habitat. Several field studies have shown that larval plankton accumulates in fronts, but have failed to give a conclusive proof that effective Lagrangian transport takes place. A few process-oriented numerical studies have lent support to the idea, but the scope of their results is limited by the two-dimensional nature of the flows considered and by the simple model used to account for the behavior of plankton. In this paper, we relax both constraints. We solve the three-dimensional Navier-Stokes equation to compute the time dependent velocity field, and we use an empirically based model for the behavior of plankton. Our results show that accumulation and transport is possible, even for larvae characterized by sustained swimming speeds that are small compared with the speed of propagation of the front. We introduce a simple model to characterize the accumulation along the front, which includes both entrainment and detrainment. The model accurately represents accumulation calculated from the numerical runs, and provide a simple tool to estimate transport under a variety of circumstances. We also investigate the spatial distribution of plankton along and across the front and show that it is very patchy and dependent on the swimming speed of plankton, with important implications for interpreting results from field experiments.

Ecological and Evolutionary Consequences of Patchiness: A Marine-Terrestrial Perspective

A quantitative description of patchiness and the assessment of its effects on ecological and evolutionary processes represents a major research focus as well as a challenge for ecologists and evolutionary biologists (e.g., Pickett and White 1985, Shorrocks and Swingland 1990, Kolasa and Pickett 1991). Patchiness is neither unique in origin nor characteristic of particular temporal or spatial scales; rather, patchiness emerges from the interactions between physical and biotic processes (Levin 1976, 1978) and is apparent at any scale of resolution. The scale dependency of patchiness and the complexity it generates calls attention to the need for new modeling approaches where spatial and temporal heterogeneity is explicitly incorporated (e.g., Hassell et al. 1991, Deutschman et al., this volume) and for new methodological tools to deal with problems of scale (e.g., Milne 1992, Garcia-Moliner et al., this volume).

Tidal mixing modulation of sea surface temperature and diatom abundance in Southern California

In the Southern California Bight a clear seasonal cycle in temperature and diatom abundance is observed, with maximum temperatures in summer and maximum diatom abundance in spring, decreasing in summer. Within this seasonal cycle of temperature and diatom abundance, there is a weak fortnightly temperature variability. Here, we show that diatom abundance has lunar as well as seasonal variability, with the highest abundance corresponding to the coldest days within the lunar cycle. This suggests that at least part of the temperature and diatom abundance variability may be due to bottom tidal mixing. To explore the effect of tidal mixing and tidal straining on the control of the thermal structure of the water column in the Southern California region, a one-dimensional, hydrodynamic numerical model is used. The model is successful in explaining the seasonal cycle in temperature and partially explains the fortnightly and monthly variability in temperature. The observed temperature minimum shows a lag of about 2–3 days, when compared with model results. This lag is probably due to the fact that the model does not include the effect of internal waves, which will be an extra source of mixing and may have an advective effect that will modify the water-column structure and the diatom distribution.