Seasonal and spatial distribution of the mesozooplankton of Southampton Water with particular reference to the contribution of copepods and barnacle larvae to pelagic carbon flux
In the past half century, a number of studies have described the general composition of the mesozooplankton of Southampton Water, highlighting aspects about the seasonality of the major components and identifying calanoid copepods and barnacle larvae as the major elements. Despite the number of studies, almost all knowledge about species composition, dominance and succession patterns of the mesozooplankton as a whole, is described from only a few studies, usually located at stations in the mid and lower estuary. It is clear that generalizations made for these stations will not reflect other parts of this estuary. Because of this, a 120 μm net-haul study comprising upper, mid and lower stations within Southampton Water was conducted over a period of 19 months, from 12/01/01 until 16/07/02, in order to critically re-evaluate the mesozooplankton community of the estuary, as well as to assess the importance of copepods and barnacle larvae to pelagic carbon fluxes. Additional biological and non-biological water column parameters were measured concurrently. A total of 144 different taxa were recorded within the zooplankton of Southampton Water during this study, with 92 identified to species, 30 to genus and 22 identified at a higher level. From these 31 were identified as holoplankton, 72 as meroplankton and 41 as tycoplankton, with 90 taxa recorded for the first time in Southampton Water. Numerically the zooplankton community was mainly composed of holoplankton forms (~69%), followed by meroplankton (~30%) and tycoplankton (~1%). Copepod nauplii were the most abundant holoplanktonic taxa, averaging 38% of all forms, followed by the calanoid Acartia spp. (31%), the cyclopoid Oithona nana (11%), the harpacticoid Euterpina acutifrons (11%) and the appendicularia Oikopleura sp. (5%). Barnacle larvae averaged 53% of the meroplanktonic forms, followed by polychaete (19%), gastropod (13%), bivalve (9%) and bryozoan larvae (3%). Harpacticoid copepods comprised 97% of the tycoplanktonic forms recorded. One unexpected finding of this study was the significant occurrence of the cyclopoid Oithona nana within the upper estuary, contrasting with previous studies where calanoids of the genus Acartia were considered the only dominant copepod form. Although present throughout the estuary, O.nana was clearly most abundant in the upper estuary where it presented a clear seasonal pattern, and was numerically the most abundant form from late-summer until early-winter, then replaced by copepod nauplii and Acartia spp. during mid-winter to late-spring, and by copepod nauplii, Acartia spp. and E.acutifrons during early to mid-summer. Barnacle larvae presented the same composition and seasonality reported in the past, with Elminius modestus the most abundant and frequent, and occurring throughout the year although it was outnumbered by Balanus crenatus from February to May. Of the remaining barnacle species found only Balanus improvisus, Semibalanus balanoides and Verruca stroemia were present in substantial numbers. Production of several copepod components was calculated, and an overall averaged production of 253.48 mg C m-3 yr-1 was estimated, with Acartia accounting for 55.6% of the production followed by E.acutifrons (16.0%), copepod nauplii (15.2%) and O.nana (13.2%). This previously unaccounted production may assist in readdressing the relatively low copepod secondary production previously estimated for Southampton Water. Production of barnacle larvae was also calculated and an overall averaged production of 32.80 mg C m-3 yr-1 was estimated, with E.modestus alone accounting for 54.7% followed by B.crenatus (35%), B.improvisus (6.7%), S.balanoides (3.1%) and V.stroemia (0.5%). Overall, production of barnacle larvae within Southampton Water is significantly lower than that of calanoid copepods contradicting previous assumptions that barnacle larvae could provide as much secondary production as calanoids. A new set of simple linear regression equations applicable to a range of crustacean zooplankton types are proposed for the preliminary estimation of production based primarily on the total number of organisms. Abundance, in conjunction with temperature, salinity and chlorophyll a pattern were also employed in the elaboration of multiple regression equations. Production values calculated by this new method were usually ±20% of the averaged value obtained by more conventional methods. When applied to an independent data set, differences of only ±7% were observed between production estimates using conventional and the new equations. The new estimated production values for barnacle larvae (meroplankton), Acartia (calanoid), Oithona (cyclopoid), Euterpina (harpacticoid) and copepod nauplii components of the mesozooplankton are integrated into an existing carbon-flux box-model for Southampton Water.