Zooplankton spatial and temporal dynamics, community composition and bentho-pelagic coupling in Comau Fjord, northern Chilean Patagonia

Abstract

Zooplankton are key players in the functioning of the marine pelagic food web of high latitude ecosystems as they constitute an essential link between primary producers and higher consumers. In addition, zooplankton provide food for benthic invertebrates, such as cold-water corals. The steep walls of Comau Fjord, northern Chilean Patagonia, harbor a diverse and rich benthic community, including dense aggregations of cold-water corals that thrive even in adverse conditions such as aragonite-depleted waters. This paradox has been attributed to a rich food supply, zooplankton being the main diet of the corals. However, the information on zooplankton seasonal and vertical distribution, together with its community composition, is still very fragmentary in Patagonian waters. Furthermore, abiotic and biotic drivers influencing the zooplankton dynamics under seasonal environmental variations (e.g. changes in solar radiation and rainfall) and their potential linkage to the corals in Comau Fjord are so far unknown. The aim of this thesis is to identify the drivers governing the zooplankton dynamics in Comau Fjord, and to understand the role of zooplankton in the bentho-pelagic coupling. To achieve this objective, the zooplankton distribution and composition were investigated both spatially and temporally. Stratified vertical hauls from surface waters to near the seabed were carried out both seasonally and during day and night by a closing Nansen net at different locations in the fjord. Subsequently, zooplankton samples were scanned with state-of-art digital imaging system (ZooScan), and zooplankton abundance, biovolume and biomass were determined for 41 zooplankton and 11 meroplankton taxa out of a total of 163,840 vignettes identified in EcoTaxa. The zooplankton availability was used to estimate spatial and temporal differences in food availability for benthic coral communities and were further compared to measured trophic markers in corals from different coral populations along the fjord. The zooplankton distribution in Comau Fjord showed a pronounce seasonality (chapter 2). Its abundance peaked in austral spring, suggesting a close link between zooplankton reproduction and phytoplankton spring bloom. However, zooplankton biomass was the highest in summer and lowest in winter, indicating that during the warmer season, bigger individuals or larger taxa developed and during the cold season the low primary production was not enough to support a high zooplankton growth. Higher zooplankton biovolume were observed in the outer part of Comau Fjord compared to the central-inner fjord and could clearly be associated with a higher chlorophyll a concentration at the fjord mouth (chapter 3). Throughout the year and along the horizontal transect, zooplankton abundance was concentrated in surface waters, while highest biovolumes were found in intermediate waters during daytime, and in surface layers during nighttime, indicating the diel vertical migration of large zooplankton (chapter 2 and 3). The community composition of zooplankton in Comau Fjord was mainly dominated by copepods, both in abundance and biovolume, followed by chaetognaths, mysids, cnidarians, and amphipods (in terms of biovolume) and meroplankton, appendicularians and ostracods (in terms of abundance) (chapter 2 and 3). Meroplankton, defined as planktonic life stages of benthic organisms, have an important role in the bentho-pelagic coupling of marine ecosystems. Meroplankton abundance concentrated in the upper 50 m of the water column (chapter 4), and was highest in spring, coinciding with higher chlorophyll a and oxygen concentrations, suggesting a close coupling between macrobenthos reproduction and meroplankton feeding with the phytoplankton spring bloom. The dominance of the different meroplankton taxa shifted seasonally, being echinoderms the most abundant in spring, gastropods in summer and autumn, and bryozoans in winter. The fatty acid composition of corals from both shallow and deep water sites revealed strong differences in their main food source (chapter 5). In shallow, fatty acid markers indicated a phytoplankton-based (dominated by diatom) diet with contribution of small copepods, while corals from the deep sites showed a dominance of calanoid copepod-related markers that may explain the higher energy stores in deep corals. However, this is not reflected by the low zooplankton assessment (chapter 2 and 3). This discrepancy may derive from ecological differences at the two different depths as well as methodological issues, both discussed in this thesis. Overall, this thesis provides novel insights into (a) the zooplankton community composition in Chilean Patagonian waters, mainly dominated by copepods, with an unprecedented level of spatial and temporal detail; (b) the main drivers governing the zooplankton dynamics, such as water column stratification, estuarine circulation or diel vertical migration; and (c) how the strong seasonal environmental variations of Comau Fjord affect not only the pelagic system (holo- and meroplankton), especially in the highly variable surface waters, but also the coral communities that populate the fjord. Furthermore, this thesis highlights the importance of zooplankton as an essential food supply to opportunistic benthic invertebrates that seasonally need to adapt to the high variations in zooplankton distribution and affect their trophic ecology. Additionally, the data clearly underscores the information gain through regular monitoring of pelagic communities in coastal ecosystems that allow to assess the natural extent of bentho-pelagic coupling as well as how anthropogenic perturbations may impact the functioning of the marine food web.